Gulf of Mexico Science Volume 19 Article 2 Number 1 Number 1

2001 Distribution and Abundance of Phytoplankton, Zooplankton, Ichthyoplankton, and Micronekton in the Deepwater Gulf of Mexico Douglas C. Biggs Texas A&M University

Patrick H. Ressler Texas A&M University

DOI: 10.18785/goms.1901.02 Follow this and additional works at: https://aquila.usm.edu/goms

Recommended Citation Biggs, D. C. and P. H. Ressler. 2001. Distribution and Abundance of Phytoplankton, Zooplankton, Ichthyoplankton, and Micronekton in the Deepwater Gulf of Mexico. Gulf of Mexico Science 19 (1). Retrieved from https://aquila.usm.edu/goms/vol19/iss1/2

This Article is brought to you for free and open access by The Aquila Digital Community. It has been accepted for inclusion in Gulf of Mexico Science by an authorized editor of The Aquila Digital Community. For more information, please contact [email protected]. Biggs and Ressler: Distribution and Abundance of Phytoplankton, Zooplankton, Ichthyo

Gu!foJMexico Science, 2001(1), pp. 7-29

Distribution and Abundance of Phytoplankton, Zooplankton, Ichthyoplankton, and Micronekton in the Deepwater Gulf of Mexico

DoUGLAS C, BIGGS AND PATRICK H. RESSLER

Expeditions in the 1960s and 1970s are the basis for the general paradigm that standing stocks and productivity of phytoplankton are both low ( <0.1 mg chl·m-3; <150 mg C.m-2·d-1) seaward of the shelf-slope break in the Gulf of Mexico. The present review supports this description of the mean (stable) state but also shows "hot spots" in primary production (>2 g C·m-2·d-1) occur when/where nutrient availability is locally enhanced seaward of the shelf-slope breal{. Recent collec­ tions with Bongo and MOCNESS nets, midwater trawls, and bioacoustic surveys of the Loop Current and associated cyclonic and anticyclonic eddies in the Gulf of Mexico show that these deepwater "hot spots" have higher stocks of zooplank­ ton and micronekton as well. The local aggregations ranged in size from coarse­ to meso- spatial scales (lOs to lOOs of kilometers) though locations of such "oa­ ses" were spatially variable along the continental margin.

hytoplankton distribution and abundance ological conveyor belt to maintain the ex­ P in Gulf of Mexico (GOM) waters has been change of pelagic species between the Carib­ reviewed at decadal intervals, first by Bjorn­ bean and the GOM (Wiseman and Sturges, berg (1971), then by Iverson (in Iverson and 1999). This conveyor does not fertilize down­ Hopkins, 1981), and most recently by Vargo stream plant plankton, however, because LC (in Vargo and Hopkins, 1990). However, most surface waters are among the most oligotro­ of the primary literature these reviewers cited phic in the world ocean. Nitrate, phosphate, focused on the continental shelf. Moreover, and other essential plant nutrients are usually Vargo, in particular, noted that much of the below the analytical detection limit ( <0.05 information for his review came from studies J.LM·l- 1) in LC inflow water from the surface to conducted prior to 1980. In fact, data collected depths of 80-90 m. The extinction coefficient, by expeditions in the 1960s and 1970s remain "k," which describes how rapidly irradiance the basis for the general paradigm that stand­ decreases with depth according to the expo­ ing stocks and productivity of phytoplankton nential equation Iz = Ia · e-kz, is usually <0.05 are both quite low seaward of the shelf-slope in LC surface water. As a consequence, the LC break in the GOM (<0.1 mg chl·m-3; <150 mg inflow into the GOM is almost swimming pool C m-2.d-1). In the present review, we will sup­ clear and therefore is deep blue in color. port that description of the mean state but we In the central and western deepwater GOM, will also show that research carried out since the standing stocks and biological productivity 1987 indicates "hot spots" in primary produc­ of the plant and animal communities living in tion (>2 g C·m-2·d-1) occur when/where nu­ the upper part of the water column are also in trient availability is locally enhanced, even in general those that might be expected in a nu­ deepwater (water depths greater than 300 m). trient-limited ecosystem. In the late 1960s, as In this review, we summarize the available evi­ part of a review of plankton productivity of the dence from the GOM that deepwater "oases" world ocean, Soviet scientists characterized the that are temporally persistent (even if they are deepwater GOM as very low in standing plank­ spatially variable) have higher stocks of zoo­ ton biomass (Bogdanov et al., 1968), with plankton and micronekton. mean primary productivity of just 100-150 mg C·m-2·d-1 (Koblenz-Mishke et al., 1970). A few DEEPWATER PHYTOPLANKTON: MEAN CONDITION years later, extensive surveys of phytoplankton chlorophyll and primary production that span The GOM is a subtropical ocean basin in the period 1964-71 were summarized by El­ which the near-surface circulation is dominat­ Sayed (1972) in atlas format as averages within ed by the anticyclonic flow of the Loop Cur­ 2° squares of latitude and longitude. These at­ rent (LC). East of 90°W, upper layer flow en­ las maps show that surface chlorophyll gener­ ters through the Yucatan Channel and leaves ally ranges 0.06-0.32 mg·m-3 in deepwater through the Florida Straits. Because this cur­ central and western GOM. There is usually a rent enters from the Caribbean, it acts as a bi- subsurface "deep chlorophyll maximum"

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(DCM) within which concentrations are 2-3 fold higher, and so the atlas reported that chlo­ rophyll in deep water could reach 21 mg·m-2 when integrated from the surface to the base of the photic zone. Most values, though, ranged 5-17 mg m-2 where water depth was greater than 2,000 m (El-Sayed, 1972). Low val­ ues of primary production ( <0.25 mg C.m-:1·lu·-1) are typical for surface waters at the mecosystems (Fitzwater et al., at the low end of the estimated range of 50- 1982). After 1982, such "clean techniques" 160 g C·m-2·yr-1 that is generally accepted for were used routinely to remeasure primary pro­ the annual gross primary production in open­ duction in the GOM. Ferguson and Sunda ocean ecosystems (Smith and Hollibaugh, (1984) reported rates of 0.11 mg C·m-3·11r-I 1993). for a LC station. Ortner eta!. (1984) measured Later studies conducted size fractionation of similar values in the LC and calculated that in­ chlorophyll and primary production in deep tegrated production rates (0-90 m) ranged water. Early data were summarized by El-Sayed from 14 mg C·m-2·hr-1 (temperature-stratified and Turner (1977). They noted that the <20- conditions) to 62 mg C·m-2-lu·-1 (after wind fLm size fraction accounted for on average 83% mixing to 110-120 m). Yoder and Mahood of the standing crop and 83% of the total pro­ (1983), who measured primary production duction. These values emphasize the impor­ from the shelf out into deep water during the tance of the nanoplankton size fraction in the Southwest Florida Shelf Ecosystern.s Study, phytoplankton community and further rein­ found that production averaged 0.1 g force the paradigm that low-nutrient surface C·m-2·d-1 in deep water outside an eddy-in­ waters are characteristically dominated by duced upwelling area. On average, then, it ap­ small-size phytoplankton and by blue-green al­ peared that remeasurements with the use of gae like Trichodesmiwn. Vargo and Hopkins clean techniques in the 1980s yielded results (1990) emphasized the importance of this that were comparable to those that were ob­ blue-green alga in the deepwater GOM, for tained during the more extensive surveys of when abundant in the top 20 m of the water the 1970s. column, Trichodesmium may have photosynthet­ In a recent review of patterns of primary ic rates of tens of milligrams of C per square productivity in tl"Ie GOM, Lohrenz eta!. (1999b) meter per day (Carpenter, 1983). After the po­ provided a plot of locations where 14C primary tential importance of phytoplankton even production measurements have been made in smaller in size than nanoplankton became the GOM. Most of these lie over the continen­ widely recognized, subsequent researchers tal shelf (water depth <200 m), and the dens­ working in the GOM and elsewhere have size est spacing is over the inner and middle shelf fractionated chlorophyll and primary produc­ off the Mississippi-Atchalafaya River. In con­ tion into pi co ( <2 11m) as well as nano (2-20 trast, Figure 1 shows the location of primary 11m), and net (>20 11m) fractions (Al-Abdul­ production measuren"Ients made in deep water kader, 1996; Gonzalez-Rodas, 1999). after Vargo's review by Texas A&M University When it became known that even low con­ and by the Universidad Nactional Autonoma centrations of trace metals can greatly depress de Mexico (UNAM) during the period 1987- measured rates of gross primary production, 1999. All of these measurements were done by biogeochemists advocated the use of trace-met­ trace-metal clean techniques. The 1990 deep­ al clean techniques to remeasure primary pro- water measurements made in support of the https://aquila.usm.edu/goms/vol19/iss1/2 2 DOI: 10.18785/goms.1901.02 Biggs and Ressler: Distribution and Abundance of Phytoplankton, Zooplankton, Ichthyo

BIGGS AND RESSLER-PLANKTON AND PRODUCTMTYIN DEEP WATER 9

Nutrient Enriched Coastal Ocean Productivity the single nwst important factor controlling (NECOP) program were reported by Biggs and the seasonal cycle in surface pign"Ient concen­ Sanchez ( 1997), and the 1987-88 measure­ tration is the depth of the mixed layer (Walsh ments were discussed by Biggs (1992). Three et al., 1989). Miiller-Karger et al. (1991) con­ dozen deepwater measurements made during cluded that, because of this dependence, an­ 1992-94 in support of the Texas-Louisiana nual cycles of algal biomass are one or more Shelf Circulation and Transport Processes months out of phase relative to the seasonal Study (LATEX) were reported by Al-Abclulkad­ cycle of sea surface temperature. er (1996) and Conzalez-Roclas (1999). The pri­

mary productivity data from UNAM stations DEEPWATER PH\'TOI'LANKTON: "HOT SPOTS" taken in summer 1997 were obtained from FROM ENTRAINMENT OF FRESHWATER Dr. Elva Escobar-Briones ([email protected]:~yl. unam.mx). Because essential plant nutrients are limit­ During LATEX, size fractionation of chlo­ ing, any process that increases the nutrient rophyll and primary production was clone concentrations available to the phytoplankton along cross-margin transects that extended in the deepwater COM will increase primary from shallow water to the shelf edge and also productivity. That freshwater inputs carry high at sampling sites along and seaward of the 200- nutrient loads is well known, but in the GOM, m isobath of the western and central COM. these high nutrient inputs are usually measur­ Ten LATEX cruises from 1992 to 1994 sampled able only close in to rivers and estuaries (Loh­ the continental margin in May (1992, 1993, renz et al., 1997, 1999a). An exception occurs, 1994), Aug. (1992, 1993, 1994), Nov. (1992, howeve1~ when surface currents set up off-shelf 1993, 1994), and Feb. (1993 only). Nowlin et flow that carries the river water seaward past al. (1998) summarized the circulation and the shelf-slope break and into deep water transport processes; phytoplankton pigment (Miiller-Karger et al., 1991). Biggs and Muell­ concentrations and species counts were re­ er-Karger (1994) combined CZCS data with ported by Neuhard (1994) and Bontempi ship data to document that high-chlorophyll (1995) and also by Al-Abdulkader (1996) and "plumes" form in the western COM when a Gonzalez-Rodas (1999). In general, the LATEX seaward-moving surface flow confluence is cre­ results support the findings of El-Sayed and ated by deepwater cyclone-anticyclone circu­ Turner (1977) that pico+nanoplankton make lation pairs. Analogous to a pair of anticlock­ up more than % of deepwater cell counts and wise-rotating and clockwise-rotating gears, accounted for >2/:J of the primary production. these circulations entrain coastal water from The exception was the "winter" cruise in Feb., the western and central GOM and draw this when diatoms of the genera LejJtocylindrus and offshore when the cyclone (anticlockwise cir­ Chaetoceros comprised >50% of phytoplankton culation) lies immediately to the north or east numbers not just in deep water but across the of the anticyclone (clockwise circulation). outer, middle, and inner shelf as well. Both cyclones and anticyclones are meso­ scale features that can be detected by the to­ DEEPWATER PHl'TOPLANKTON: SEASONAL pography of the 15°C isotherm. This isotherm CHANGES is domed upward in the cyclones and pushed locally deep within the anticyclones. Both types Pigment concentration at the surface in the of features can now be located with satellite deepwater GOM undergoes a well-defined sea­ altimetry as well because GOM cold-core ed­ sonal cycle that is generally synchronous dies (15°C isotherm domed) have 10-20-cm lo­ throughout the region. Miiller-Karger et al. cal depressions in sea surface height, whereas (1991) and Melo-Gonalzez et al. (2000) re­ warm-core eddies (15°C isotherm pushed lo­ viewed monthly climatologies of near-surface cally deep) have 20-70-cm local elevations in phytoplankton pigment concentration from sea surface height (Leben et al., 1993). As one multiyear series of coastal zone color scanner recent example, Figure 2 shows dynamic to­ (CZCS) images for the period 1978-86. They pography, gridded upper layer geostrophic ve­ reported that highest surface concentrations locity, surface salinity and surface chlorophyll of chlorophyll occur between Dec. and Feb. concentrations over deep water of the north­ and lowest values occur between May and july. east GOM in midsummer 1997. Low-salinity There is only about 3-fold variation between Mississippi River water was entrained into the the lowest ( ~0.06 mg·m-3) and highest (0.2 flow confluence created by a gradient of >80 mg·m-3) deepwater surface pigment concen­ dyn em in geopotential anomaly between be­ trations, however. Model simulations show that tween a cyclone located to the north-northeast

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.._.,· ..

75 cm/s '<:·:

-86 -84 Longitude

Longitude Longitude

0.00 0.40 0.80 1.20 1.60 2.00 Fig. 2. (A) Dynamic topography (em, 0 m relative to 800 m) of the deepwater GulfCet II focal area, as determined from 107 hydrographic stations made on R/V Gyre cruise 97G08. (B) Gridded upper layer geostrophic velocity (0 m relative to 800 m) computed from the dynamic topography data in A. (C) Sea surface salinity map, superimposed on ship track lines of 97G08. (D) Sea surface chlorophyll (mg·m-3), superimposed on ship track lines. All four figures from Chapter 2 of GulfCet II final report (Davis et al., 2000).

of a LC eddy. Note that low-salinity patches of respond spatially to the patches of lowest sur­ river water were wrapped anticlockwise around face salinity. the periphery of the cyclone. A comparison of As a second example, Figure 3 shows sea sur­ the salinity and chlorophyll fields shows that face height anomaly, surface salinity, and sur­ surface chlorophyll concentrations in this river face chlorophyll over the same region the next water reached 2.0 mg·m-3 and that, especially summer, in Aug. 1998. This time, there is no in the concentration range 0.1-0.4 mg·m-3, well-developed cyclone-anticyclone modon the patches of highest surface chlorophyll cor- pair. Rather, it is the clockwise circulation https://aquila.usm.edu/goms/vol19/iss1/2 4 DOI: 10.18785/goms.1901.02 Biggs and Ressler: Distribution and Abundance of Phytoplankton, Zooplankton, Ichthyo

BIGGS AND RESSLER-PLANKTON AND PRODUCTIVITY IN DEEP WATER 11

30°N

29°N

28°N

2TN """"'~WJ.LJ...L.I.....::.....~.~..l..l..l.~~(.,4/,/,~....I..J,;... 90°W 84°W 83°W 82°W

31 ON

30°N

29°N

28°N

2TN 90°W 89°W 88°W 8TW 86°W

Fig. 3. (Top) Sea surface height anomaly for water depths >200 m from satellite altimeter data for the NEGOM study area for 29 July 1998 (hindcast data). (Middle) Salinity at ~3 m from thermosalinograph 3 observations on NEGOM cruise N3, 26 July-6 Aug. 1998. (Bottom) Chlorophyll (mg·m- ) at ~3 m calcu­ lated from flow-through fluorescence on NEGOM cruise N3. All from NEGOM annual report, year 2 (loch­ ens and Nowlin, 1999). Shading indicates patches of low-salinity, high-chlorophyll river water being en­ trained anticyclonically around the warm slope eddy centered over deepwater in DeSoto Canyon.

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12 GULF OF MEXICO SCIENCE, 2001, VOL. 19(1)

Fig. 4. Annual mean chlorophyll concentration in the Gulf of Mexico (mg·m-3), composited using all available SeaWiFS data Jan. 1998-Dec. 1999. Note "halo" oflocally high pigment concentration (light gray color) that outlines the periphery of the Loop Current. SeaWiFS data are courtesy of Orbimage and NASA; data were collected and processed by Frank 1\thillel~Karger and annual mean was composited by Andrew Remsen (both at College of Marine Science, University of South Florida).

around the periphery of a small anticyclone clonic eddies are zones of locally high vertical that was located close off the Mississippi River shear. Lee et al. (1991) have shown that me­ delta that has entrained river waters eastward anders and eddies in the Gulf of Mexico are along its edge. In the periphery of the anticy­ often marked by local aggregations of phyto­ clone, patches of low-salinity, high-chlorophyll plankton, and elevated fish stocks appear to waters got transported from the inner shelf concentrate in such areas (Atkinson and Tar­ eastward across the continental margin to gett, 1983). The presence of multiple cyclonic deepwater depths >500 m (see also MiUler­ and anticyclonic features in the GOM can re­ Karger, 2000). Note that the two irregular­ sult in strong frontal gradients between these shaped patches of high chlorophyll (>0.6 features. 3 mg·m- ) seaward of the 200-m isobath between In the CZCS ocean color climatology from 86° and 88°W correspond, spatially, to patches 1978-1986 (MiUler-Karger et al., 1991) and in where surface salinity is <31. imagery from the current generation ocean color sensor (the Sea Wide-Field Scanner, or DEEPWATER PHYTOPLANKTON: "HOT SPOTS" SeaWiFS, in orbit since Oct. 1997), the periph­ FROM CROSS-ISOPYCNAL MIXING eries of the LC and of the anticyclonic LC ed­ Recent fieldwork has shown these tnesoscale dies (LCEs) of diameter 200-300 km that are oceanographic features have additional im­ shed from the LC are often seen to be outlined pacts upon deepwater plankton and micronek­ by surface pigment concentrations that are 2- ton com1nunities. Locally, high nutrients are 3-fold higher than the extremely low concen­ also introduced to the surface of deepwater trations (0.04-0.06 mg·m-3 ) in the interior of ocean regions at eddy edges where there is en­ these circulations. Figure 4, in which a "halo" hanced vertical mixing. In fact, the periphery of locally high chlorophyll standing stock can region of high-velocity surface currents that be seen to encircle the periphery of the LC in surrounds both the cyclonic and the anticy- this annual mean composite, is one such ex- https://aquila.usm.edu/goms/vol19/iss1/2 6 DOI: 10.18785/goms.1901.02 Biggs and Ressler: Distribution and Abundance of Phytoplankton, Zooplankton, Ichthyo BIGGS AND RESSLER-PLANKTON AND PRODUCTIVITY IN DEEP '"TATER 13

30

29

28

27

26

-97 -96 -95 -94 -93 -92 -91 -90 -89

199 201 203 205 81 79 38 36 34 29 196 200 202 168 206 80 41 37 35 33 0

50

-E1oo !

150

200 ~~~~~~~~~~~~~~~~~~~~~~~~~~~_u~~~~~~u 0 50 100 150 200 250 300 350 400 450 500 550 600 Distance along cruise track (km)

Fig. 5. (Top) Cruise track and station locations for LATEX hydrographic survey H05, April-May 1993. (Bottom) Vertical contours of bottle nitrate (fJ,J'vl·l- 1) along 200-m isobath during cruise H05. Dots indicate bottle trip depths. Both from LATEX data report (Jochens et a!., 1996).

ample. Two other examples from recent field­ 36, 37, and 38 on LATEX hydrographic survey work are presented as Figures 5 and 6. H05, nitrate concentrations >0.5 f1M·l- 1 oc­ Figure 5 shows a hot spot of anomalously cm-red at the surface, just south of a strong high nitrate concentration in surface waters surface front where salinity increased from between 91 o and 92°W along the 200-m isobath 32.0 to 36.3. This hot spot of nitrate apparently that was encountered in May 1993. At stations arose from strong vertical shear that developed

Published by The Aquila Digital Community, 2001 7 Gulf of Mexico Science, Vol. 19 [2001], No. 1, Art. 2 14 GULF OF MEXICO SCIENCE, 2001, VOL. 19(1)

100km --100mies Integrated productivity, Aug. 1993 Integrated productivity, Nov. 1994

TOPEX/ERS Analysis Aug 1 1993 TOPEX/ERS Analysis Nov 8 1994

Fig. 6. Deepwater hot spots of primary productivity (>2 g C.m-2·d-1) occurred on the LATEX conti­ nental margin in Aug. 1993 and Nov. 1994 at stations in the northern periphery of LCE-W and LCE-Y Productivity maps are from Gonzalez-Rodas (1999); triangles show location of the eight or nine primary productivity stations done each cruise. SSH anomaly maps ti·om University of Colorado (http://www­ ccar.colorado.edu/ ~realtime/ gom-historicaLssh/) are marked with stars to show the location of the highest measured 14C productivity in relation to LCE periphery.

in this frontal zone, for the surface salinity and near-surface chlorophyll at station 37 ranged silicate data and the vertical contours shown in from 0.4 to 0.5 mg·m-3, or 3-fold higher than Figure 5 strongly suggest that it was fueled by the concentrations of 0.15-0.17 mg·m-3 at sta­ cross-isopycnal vertical mixing from below tion 83 west of the hot spot. Al-Abdulkader's rather than from entrainment of freshwater data show that primary productivity in the up­ fro1n the Atchafalaya Bay or Mississippi River per 50 m of the hot spot ranged from 0.2 to to the north and east. Farther west along the 0.3 mg C·m-3 ·lw-1• Integrated to the 0.2% 200-m isobath, an anticyclone (LCE "V") was light depth and assuming that photosynthesis interacting with the continental margin. Note proceeds 12 hr per day in May, this is a pro­ as well from Figure 5 that the extremely low duction of 220 mg C· m - 2• d -l. This is 1. 4 times nutrient interior of the eddy was apparently higher than the measured production inte­ drawn onshore between stations 207 and 210. grated to the same irradiance level at his sta­ Al-Abdulkader (1996) measured chlorophyll tion 83 (158 mg C·m-2·d-1). At station 88 in stocks and primary productivity at station 37 the northeast periphery of LCE "V," locally within the hot spot of anomalously high sur­ low salinity surface water was present (33.6- face nitrate and at station 83 some 140 km to 33.8 in the upper 10m). This surface water was the west along the 200-m isobath and also far­ low in nitrate, and near-surface chlorophyll ther west at station 88 at the deepwater end of concentrations in it were similar to those at sta­ LATEX line 4, which reached the northeast pe­ tion 83, but high silicate levels in the upper 10 riphery of LCE "V." These data show that m at station 88 indicate this low-salinity cap was https://aquila.usm.edu/goms/vol19/iss1/2 8 DOI: 10.18785/goms.1901.02 Biggs and Ressler: Distribution and Abundance of Phytoplankton, Zooplankton, Ichthyo BIGGS AND RESSLER-PLANKTON AND PRODUCTIVITY IN DEEP WATER 15

probably entrained Mississippi River outflow. associated with these anticyclones represent ar­ Data from Al-Abdulkader's dissertation show eas of higher biological productivity. that primary productivity in the low-salinity Subsurface sampling of cyclonic GOM ed­ surface water was locally high (0.3-0.4 mg dies from ships showed a highly predictable C.m-3 • he 1) and that, even below this low-salin­ negative first-order relationship between tem­ ity layer, productivity averaged 0.16 mg perature <22°C and nitrate concentration. C·m-3 ·hc1 to a depth of 100 m. vVhen inte­ Temperature could thus be used as a proxy for grated to the 0.2% I" depth, this is a produc­ nitrate concentration, and in particular the tion of226 mg Gm-2·d-I, equivalent to that in depth of the 19°C isotherm was a good esti­ the nitrate "hot spot." mation of the depth of the 10 [LM·l- 1 nitrate A recent dissertation by Gonzalez-Rodas concentration (Biggs et al., 1988). vVithin one (1999) summarized primary productivity mea­ cyclone sampled in 1996, the nitracline was surements on six subsequent LATEX cruises. domed 40-60 m shallower than within the LCE Figure 6 shows Gonzalez-Rodas' summary of that was sampled concurrently (see Zimmer­ integrated primary productivity for the LATEX man and Biggs, 1999: fig. 6). Because this dom­ continental margin on two of these cruises, in ing facilitated a higher flux of new nitrogen Aug. 1993 and Nov. 1994. Note that hot spots into surface waters in cyclone than in anticy­ in deepwater primary production (>2 g clone, the DCM was locally shallower and chlo­ C·m-2·d-1) were present near 27.5°N and 92"W rophyll reached higher maximum concentra­ on both cruises. In summer 1993, the northern tion in the cyclone than in the LCE. Because edge of LCE-vV was interacting with the conti­ this resulted in higher standing stocks of chlo­ nental margin between 91° and 93°\>\T; the lo­ rophyll in the upper 100m in the cyclone, the cally high shear there apparently fueled a re­ cyclones are generally regarded as biological gion of anomalously high deepwater primary "oases," whereas the interior of the LCEs are production. This eddy had a diameter of some biological "deserts." 250 km, and at the location where the produc­ At six hydrographic stations made during a tivity was measured, the geopotential anomaly survey of a mesoscale cyclonic eddy that was was about +20 em and current speeds were centered near 26°N and 94°W in Nov. 1987, about 60 cm·s-1 (see Gonzalez-Rodas, 1999: ta­ integrated chlorophyll standing stock averaged ble 5). In fall 1994, the northern edge of an­ 38 + 9 mg·m-2 (Biggs et al., 1988), or 2-3 other anticyclone, LCE-Y, was interacting with times greater than the mean for the oceanic the continental margin again between 91 o and GOM. Primary productivity averaged 12 mg 92°W. This eddy was even larger in diameter C·m-3·d-1 in the upper 10 m, and integrated (320 km) and presented a geopotential anom­ production to the 1% light level was equal to aly of +36 em (from Gonzalez-Rodas, 1999: ta­ 250 mg Gm-2·d-1 (Biggs 1992), or double the ble 5). On four other cruises, LCEs were too mean of 100-150 mg C·m-2 ·d-1 reported by far offshore to be sampled and deepwater pri­ Koblenz-Mishke et al. (1970). Similarly, Yoder mary productivity along the LATEX margin av­ and Mahood (1983) reported that, for stations eraged <0.3 g C·m-2·d-1 (Gonzalez-Rodas, located seaward of the 200-m isobath off the 1999). West Florida Shelf within an area of eddy-in­ duced upwelling, the top of the nitracline DEEPWATER PHYTOPLANKTON: "HoT SPOTS" domed to depths of just 40-60 m below the FROM MESOSCALE DIVERGENCE surface. They measured the average water col­ umn production there at 0.6 g C·m-2 ·d-I, Because the interiors of the anticyclones are whereas for three other stations located out­ areas of convergence, the upper 100 n1 or so side the eddy-induced upwelling area, produc­ of the water column in both LC and LCEs are tion averaged 0.1 g C·m-2·d-1 (Yoder and Ma­ areas in which surface waters are infrequently hood, 1983). Thus, Yoder and Mahood con­ renewed and so they are impoverished in ni­ cluded that subsurface upwelling may enhance trogen and phosphorus nutrients. The interi­ deepwater phytoplankton primary production ors of these regions of convergence are gen­ by as much as 6-fold. Subsequent studies of cy­ erally regarded as biological "ocean deserts." clonic gyre formation off the southwest Florida Measurements of chlorophyll standing stocks, Shelf found that a cold recirculation, approx­ primary productivity, and zooplankton stand­ imately 200 km in size, develops off the Dry ing stocks within an LCE sampled in 1988 are Tortugas when the LC flow overshoots the en­ in good agreement with this premise (Biggs, try to the Straits of Florida and that this per­ 1992). However, the cyclonic cold-core eddies sists over time scales of about 100 d (Lee et al., (local areas of divergence) that are frequently 1994). Fratantoni et al. (1998) showed how this

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cyclone grows from instabilities along the east­ plankton standing stocks in the tropical oligo­ ern edge of the LC. This so-called "Tortugas trophic Caribbean Sea were almost always low Gyre" formation provides enhanced food sup­ and did not exceed 10 ml wet displacement ply, retention, and shoreward transports for volume (WDV) per 100m3 in waters offshore successful recruitment oflocally spawned snap­ of the shelf-slope break, GOM stocks exhibit­ per and grouper larvae in the western and low­ ed more seasonal, interannual, and spatial var­ er Florida Keys. iabilit:y, with biomass levels as high as 35 ml In sununary, the GOM is oligotrophic in 11\TDV per 100 m~ (range <5-35). Also, Hop­ general, but mechanisms exist that elevate pri­ kins and Lancraft (1984), who compared in­ mary production on smaller space and tin'le tegrated wet weight biomass of zooplankton scales. These mechanisms serve to increase and micronekton in three tropical-subu-opical heterogeneity in what is otherwise classically locations (Caribbean Sea, GOM, and Pacific defined as a "stable" ecosystem. When/where Ocean near Hawaii), found that the GOM was anticyclonic and cyclonic hydrographic fea­ the highest in terms of zooplankton and inter­ tures occur over deepwater and especially mediate in rank (above the Caribbean) in where they interact with the continental mar­ terms of micronekton. If gelatinous plankton gin, they are expected to play an important were included in the micronekton biomass role in determining biogeographic patterns comparison, the GOM then ranked highest of and controlling primary productivity. all three locations in both categories. Finally, although studies of GOM biomass do generally 3 DEEPWATER ZOOPLANKTON, ICHTHYOPLANKTON, reveal low standing stocks ( <5 ml·1 00 m- ), AND MICRONEKTON: MEAL'! CONDITION reported estimates can vary by a factor of 10 or more from the minima within a given study The deepwater GOM has been considered a (Biggs et al., 1988; Richards et al., 1993; Wor­ biologically impoverished ocean for zooplank­ muth et al., 2000) to values comparable to ton, ichthyoplankton, and micronekton be­ standing crops found in upwelling regions of cause on average the standing stocks of plank­ other oceans (14---75 ml·100 m-3, as summa­ ton and fish seaward of the shelfbreak are low­ rized by Austin and Jones, 1974). er than those found in temperate and higher The presence of sizable populations of apex latitude regions. Soviet-Cuban fisheries inves­ predators in the deepwater GOM also contra­ tigations in the 1 960s reported that zooplank­ dicts the paradigm of uniformly low secondary ton standing stocks were low across much of production. The larvae and adults of tuna, the GOM (Bogdanov et al., 1968; Khromov, swordfish, mackerel, and other nekton of im­ 1969a), and subsequent reviews by Hopkins portance to commercial and recreational fish­ have reinforced this perception (Iverson and eries are found in the deepwater GOM (Vargo Hopkins 1981; Vargo and Hopkins, 1990). In and Hopkins, 1990; O'Bannon, 1999). Com­ fact, in several biologically important ways, the mercial landings of adult yellowfin tuna alone GOM resembles other oligotrophic subtropical exceeded 3.7 million pounds (value >$9 mil­ oceans. The zooplankton and micronekton lion) in 1998 (National Marine Fisheries Ser­ fauna of the deepwater GOM are similar in en­ vice Annual Commercial Landing Statistics, ergy content, taxonomic composition, and http:/ /www.st.nmfs.govI commercial/landings/ food habits to those of other low-latitude annuaLlandings.html). The deepwater GOM is oceans (Stickney and Torres, 1989; Hopkins also habitat for substantial populations of ma­ and Gartner, 1992; Hopkins et al., 1994, 1996), rine mammals, sea turtles, and seabirds (Mul­ and the ichthyoplankton fauna of the GOM lin et al., 1994; Davis et al., 1998; Weller et al., have been grouped along with those of the 2000). In fact, the same cyclones and the fron­ western tropical Atlantic Ocean and Caribbean tal zones of both cyclonic and anticyclonic eel­ Sea (Richards, 1990). dies shown to support enriched zooplankton Relegating secondary production in the and micronekton biomass (Wormuth et al., GOM to oligotrophic status is nevertheless an 2000) have been identified as deepwater con­ oversimplification because the generally low centrating mechanisms for apex predators standing stock levels are not uniformly low but such as fish and marine mammals (Lamkin, are instead punctuated by spatial and temporal 1997; Biggs et al., 2000; Davis et al., 2000). In variation greater than that found in most other this review, we show that anticyclonic and cy­ oligotrophic oceans. This variability may be clonic hydrographic features play an important manifested as spatial "hot spots" and temporal role in determining biogeographic patterns of peaks in biomass. For example, Khromov and controlling secondary productivity in ( 1969a, 1969b) reported that, whereas zoo- deepwater of the GOM. https://aquila.usm.edu/goms/vol19/iss1/2 10 DOI: 10.18785/goms.1901.02 Biggs and Ressler: Distribution and Abundance of Phytoplankton, Zooplankton, Ichthyo

BIGGS AND RESSLER-PLANKTON AND PRODUCTIVITY IN DEEP WATER 17

TABLE l. Chronology of previous reviews of the zooplankton, ichthyoplankton, and micronekton of the Gulf of Mexico.

Year Author Synopsis 1954 Galtsoff (editor) An edited volume containing reviews of GOM zooplankton and micronekton; first m'\ior synthesis 1970 Pequegnat and Chace ( ed­ Texas A&l'vl University oceanographic studies. Vol. 1; contains re­ itors) views of some groups of zooplankton and micronekton; em­ phasis is on benthic/demersal rather than pelagic tecology of the Gulf of Mexico published in Ame1ican Zoologist 1990 Vargo and Hopkins Hopkins' portion reviewed zooplankton and micronekton + ichthyoplankton. The area of interest was Florida south of the Keys and the deepwater GOM to the west of the Florida coast in MMS's Eastern Planning Area

DEEPWATER ZooPLANKTON, ICHTHYOPLANKTON, (1970) edited a volume on the biology of the AND MICRONEKTON: PREVIOUS REVIEWS GOM that contained a historical overview, lo­ cations, and discussion of investigations of wa­ Several mcopepods, cladocera, os­ tailed information was available, the general tracods, mysids, amphipods, isopods, euphau­ conclusion was that there was still much to be siids, decapods, chaetognaths, hemichordates, learned about the GOM zooplankton/micro­ urochordates, and cephalochordates. Bjorn­ nekton community. In fact, Moore concluded berg concluded that the copepods and chae­ that on balance "next to nothing of the zoo­ tognaths were the best studied groups, re­ plankton of the Gulf of Mexico'' was known at marked that much of the study of GOM zoo­ the time. plankton to date had been concentrated in Sixteen years later, Pequegnat and Chace coastal waters and the Florida Current, and fi-

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nally noted the need for large-scale, coordinat­ ergy (DOE). The zooplankton were studied off ed study of the zooplankton in shelf and oce­ Mobile Bay (29°N, 88°W) and off Tampa Bay anic waters of the GOM. In the same volume, (27°38'N, 85°34'W). The investigators were Rass reviewed deep-sea fish fauna (members of able to observe taxonomic composition and the rnicronekton community). Rass provided a biomass levels as a function of depth and time, list of 203 species fron'l the GOM and estimat­ although the sampling strategy did not allow ed that deepwater fish represented about one­ them to completely resolve diurnal or seasonal third of the total number of fish species in the trends. Hopkins also summarized his own Na­ open GOM. tional Science Foundation-funded trophodyn­ In a compendium entitled "A Summary of amic study of zooplankton and micronekton in Knowledge of the Eastern Gulf of Mexico," the upper l ,000 m at a station in the eastern Hopkins (1973) reviewed GOM zooplankton. central GOM (27"N, 86°W). Diurnal patterns Work in estuarine and coastal systems had of zooplankton numbers and biomass were been increasing, but Hopkins noted little work studied with trawling, net tows, and bottle sam­ had been published on zooplankton in the pling. Vertical migration was documented for oceanic GOM. However, knowledge of eastern a "significant portion of the zooplankton and GOM physical oceanography had increased micronekton in the east-central Gulf." Hop­ considerably, and its potential biological effects kins estimated that the zooplankton biomass at were pointed out by Hopkins. The LC and as­ this reference station turned over once every sociated upwelling were cited as the most im­ 30-90 d, supported by the relatively low pri­ portant factors affecting plankton production mary production in the oligotrophic open in the oceanic GOM, whereas in coastal areas, GOM. Some inferences were made about tro­ runoff from terrestrial sources and seasonal phic interactions on the basis of the data col­ temperature changes were the most important. lected there, and Hopkins included a list of Biomass was known to be low in the oligot:ro­ important zooplanktonic and micronektonic phic open GOM and was thought to vary sea­ predators and prey in the system. sonally with the movement of the LC. The use From the studies cited in Hopkins' review of zooplanktonic indicator species as water for the 1979 symposium, the temporally and mass tracers was mentioned in this review, as spatially patchy nature of the zooplankton and well as the ongoing plankton collections that micronekton had become evident. Hopkins were taking place as part of the EGMEX (East­ emphasized the general lack of basic physio­ ern Gulf of Mexico) program. Hopkins' own logical data for GOM zooplankton, though, quantitative studies of biomass and taxonomic which he argued was urgently needed to better composition of zooplankton and micronekton understand the flow of energy and/ or pollut­ in the eastern central GOM were mentioned ants through the deepwater ecosystem. as "in progress." Briggs reviewed midwater fishes of the GOM in the same volume, but he In 1987, a special session on the ecology of noted that the ichthyofauna of waters overlying the GOM was held at the annual meeting of the continental slope and abyssal plain were the American Society of Zoologists. In 1990, still not well known. selected papers from that session were pub­ In 1981, a review of GOM phytoplankton lished in an issue of the journal Anwican Zo­ and zooplankton by Iverson and Hopkins was ologist. Darnell and Defenbaugh (1990) re­ included in the proceedings of a 1979 sympo­ viewed the history of environmental research sium on "Environmental Research Needs in in the GOM, noting that in the 15 yr preceding the Gulf of Mexico." Hopkins' section on zoo­ their review, federal agencies (most notably the plankton reviewed work on the shelf and slope Department of the Interior) had spent more and in the open GOM subsequent to previous than $75 million in research studies of the reviews of GOM zooplankton, micronekton, northern GOM. As had previous reviewers of and ichthyoplankton. Hopkins noted that, ex­ the GOM zooplankton/micronekton field of cept for published work on zooplankton tax­ study, these authors reported that much of the onomy, much of the research clone remained results of GOM research remained "locked up in "gray literature" (government reports and in the various technical reports submitted to theses/ dissertations). However~ Hopkins fea­ the sponsoring agencies, and only a small frac­ tured several major research programs that tion [had] appeared in the professional jour­ sampled zooplankton in water depths of 200 m nal literature." However, although this review or greater in the review, including Ocean provided a list of early historical investigations Thermal Energy Conversion ( OTEC), a pro­ of the GOM and of major interdisciplinary in­ gram sponsored by the U.S. Department of En- vestigations since 1960, the bulk of these stud- https://aquila.usm.edu/goms/vol19/iss1/2 12 DOI: 10.18785/goms.1901.02 Biggs and Ressler: Distribution and Abundance of Phytoplankton, Zooplankton, Ichthyo BIGGS AND RESSLER-PLANKTON AND PRODUCTIVITY IN DEEP WATER 19

ies had been targeted to the continental shelf community but become more numerous closer and not to deep water. to shore (Vargo and Hopkins, 1990). In terms In 1990, Vargo and Hopkins provided are­ of feeding, the zooplankton community in­ view of COM phytoplankton, zooplankton, and cludes herbivorous, cletrivorous, and omnivo­ ichthyoplankton in a report to the U.S. Min­ rous members (Hopkins, 1982). The top three erals Management Service (MMS). The area of groups of deepwater micronekton in order of interest was South Florida, mostly south of the biomass are scyphomeclusae, fish (myctophicls Florida Keys but also including the deepwater and gonostomaticls), and crustaceans (cleca­ GOIVI to the west of the Florida coast (in pocls and euphausiids) (Hopkins and Lancraft, MMS's Eastern Planning Area). Hopkins' por­ 1984). Zooplanktonic crustaceans comprise tion of the review included COM hydrography the greater part of the diet of micronektonic and circulation relevant to zooplankton, ichth­ miclwater fishes (Hopkins and Baird, 1977; yoplankton, and micronekton populations, as Hopkins et al., 1996) and crustaceans (Hop­ well as tabular data and a discussion regarding kins et al., 1994), and gelatinous carnivores are the taxonomic dominants and seasonal trends also known to be important zooplanktonic in abundance and biomass in COM waters predators (Biggs et al., 1984; Vargo and Hop­ deeper than 200 m. kins, 1990). Further, areas of enriched deep­ water zooplankton biomass have been shown to be correlated with increased abundance of DEEPWATER ZOOPLANKTON, lCHTHYOPLANKTON, squid paralarvae and myctophicl fishes (Wor­ Al'ID MICRONEKTON: SYSTE.MATICS STUDIES muth et al., 2000). The major components of Many studies of the diverse zooplankton, the deepwater ichthyoplankton community are ichthyoplankton, and micronekton of the larval myctophicls, gonostomaticls, mackerel, COM have concentrated on the ecology, biol­ tuna, and flyingfishes (Vargo and Hopkins, ogy, or systematics of one particular species or 1990; Sanvicente-Anorve et al., 1998). The group of organisms. Because a table of these presence of increased abundance of larval fish works would make the length of this review un­ in areas of enrichecl.zooplankton biomass im­ necessarily long, we have archived them chro­ plies that their diets include zooplankton (Go­ nologically by author with summary descrip­ voni et al., 1989; Lamkin, 1997). However, the tion of subject at: http:/ /www-ocean.tamu. available information on the feeding habits of eclu/ ~biggs/ deepwater-reviewI .1 Although ichthyoplankton is limited, except as the cate­ the scope of these individual works may be nar­ gory overlaps with micronekton and zooplank­ row, in ensemble they are very important to an ton. understanding of COM zooplankton, micro­ nekton, and ichthyoplankon communities. DEEPWATER ZOOPLANKTON, ICHTHYOPLANKTON, Such research provides the means to identify AND MICRONEKTON: BIOMASS AND ABUNDANCE and enumerate specimens found in collected samples; without knowing "who" is there, we The standing stock biomass of zooplankton, cannot hope to understand the COM as a sys­ micronekton, and ichthyoplankton in the tem. To understand the flow of energy and nu­ COM has been observed to vary in both space trients through the deepwater biological sys­ and time, but despite numerous studies on the tem, Hopkins (1982) has argued, knowledge of ecology and systematics of particular taxonom­ taxon-specific trophic interactions is often ic groups, much less work has been clone to helpful. Thus, we believe this chronology will determine the scales of the variability at the be of value because these works provide the coarse- to mesoscale level and how these de­ taxon-specific ecological information needed termine the patterns in biomass over time. to interpret studies of biomass and abundance Most of the work has been clone by traditional and to allow the identification of species or net sampling techniques: a survey of bulk bio­ groups of particular importance. mass values from tqe literature reveals up to In brief, the dominant groups of COM deep­ 10-folcl and higher ,;ariability in standing stock water zooplankton in terms of biomass are hol­ levels (see Table 2). oplanktonic calanoicl copepocls, euphausiicls, Figure 7 includes two maps showing the lo­ and chaetognaths; meroplanktonic larvae are cations of major collections of plankton bio­ "relatively scarce in the oceanic" zooplankton mass data. Despite fairly extensive sampling coverage in many deepwater parts of the COM over the last 30-oclcl years, though, there has 1 A hard copy can also be obtained by contacting been no overall summary of the biomass re­ DCB at the address given at the end of this article. sults. There have, however, been numerous

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TABLE 2. Chronology of previous estimates of plankton standing stock in the deepwater GOM (biomass as milliliters wet displacement volume per 100m3)."

Year Autl10r Synopsis Biomass 1958 Arnold GOM-wide, upper 10 m, silk mesh in metal tube, horizontal 5-6 tows 1958 Arnold GOM-wide, upper 10m, all-metal sampler, horizonal tows 11-13 1969h Khromov GOM-wide, vertical hauls, upper 100m, silk Juday meter <5-35 nets with 38 meshes per inch (0.5-mm aperlure); with "inedible forms removed" 1973 Hopkins In review article, mentions biomass estimates for the eastern 1-10 central GOM that were obtained during EGJVIEX (eastern Gulf of Mexico) investigations 1976 Houde eta!. Eastern GOM from multiple years and seasons, upper 200 2-10 m, 51-em-diameter bongo nets with 333-f.Lm mesh, oblique hauls 1981 Iverson and Hopkins Tampa OTEC site in eastern GOM, upper 200m, 0.75-m 6 open nets with 202-f.Lm mesh, vertical and oblique hauls; average value reported here 1988 Biggs eta!. ·western GOM, upper 100 m, open meter nets with 333-f.Lm 4--40 mesh, oblique hauls during 2 mo (April and Nov.) of the sante year 1989 Richards et a!. Northeast GOM, upper 200m, 51-em-diameter bongo nets 2-12 with 333-f.Lm mesh, oblique hauls, data from SEAlvlAP program 1991 Grimes and Finucane Front between Mississippi plume and ocean wate1~ neuston 1-12 tows, 947-f.Lm mesh, horizontal tows 1992 Biggs Y,Testern GOM, upper 200 m, open 70-cm-diameter bongo 4--6 nets with 333-f.Lm mesh oblique hauls; range of average day-night values is shown 1993 Richards et a!. Northeast GOM, upper 200 m, 51-em-diameter bongo nets 2-33 with 333-f.Lm mesh, oblique hauls; data from SEMIAP program 1997 Biggs eta!. \.Yestern GOM, upper 100 m, open meter nets with 333-f.Lm 4--9 rnesh, oblique hauls 1997 Lamkin Upper 200 m, 51-em-diameter bongo nets with 333-f.Lm 10-13 mesh, oblique hauls, data from SEMIAP program; range using averages for the eastern and eastern GOM (respec- tively) is shown 1999 Zimmerman and Biggs Central GOJ\<1, various depth intervals in the upper 125 m, 4--32 1/4-m2 mouth area MOCNESS with 333-f.Lm-mesh nets 2000 Davis et a!., Vol. III: Northeast GOM during two different years, various depth in- <0.1-33 data appendix tervals in the upper 400 m, 1-m2 mouth area MOCNESS with 333-f.Lm-mesh nets

a Notes: Direct comparisons of biomass values arc difficult because of differences in gear, sampling technique, and measurement methods. The values above are a sampling of those values reported in wet displacement volume per volume of semvater or similar, with equipment and sampling technique as noted. Volume units ·were converted as necessary into m1·100 m-3• The implicit assumption is that these bulk values are useful in describing the overall biomass of various sizes and kinds of zooplankton, ichthyoplankton, and micronekton in the deepwater GO:M. b Values in this paper \Vere originally reported as g/m\ but a footnote iudicated that they were volume values that had been convet·ted to weights by a.,suming a zooplankton "specific weight" of ~1.

publications and analyses of the amount, com­ Gasca et al., 1995; Zimmerman and Biggs, position, and variability of the biomass at par­ 1999; Wormuth et al., 2000). ticular locations in the deepwater GOM (Com­ mins and Horne, 1979; Flock and Hopkins, DEEPWATER ZoOPLANKTON, ICHTHYOPLANKTON, 1981; Hopkins, 1982; Hopkins and Lancraft, AND MICRONEKTON: SPATIAL AND TEMPORAL 1984) and regions (Houde and Chitty, 1976; VARIABILITY Houde et al., 1976, 1979; Cummings, 1984; Biggs et al., 1988, 1997; Richards et al., 1989, The analyses that are available indicate that 1993; Grimes and Finucane, 1991; Biggs, 1992; whereas overall biomass levels are low, there is https://aquila.usm.edu/goms/vol19/iss1/2 14 DOI: 10.18785/goms.1901.02 Biggs and Ressler: Distribution and Abundance of Phytoplankton, Zooplankton, Ichthyo BIGGS AND RESSLER-PLANKTON AND PRODUCTIVITY IN DEEP WATER 21

mass may also result from turnover of the deepwater zooplankton standing stock, esti­ mated at 30-90 d for zooplankton in the east­ ern GOM (Iverson and Hopkins, 1981). The biomass in a given depth interval can vary on the time scale of a day by a factor of 2 or more because of diel vertical migration (see Vargo and Hopkins, 1990; see also Biggs eta!., 1988; \1\Tormuth et a!., 2000). lchthyoplankton distri­ butions are especially variable, for 1nany taxa exhibit pronounced seasonality and year-to­

Longitude year variation in abundance. Much of this var­ iation appears tied to length and time of year of spawning (Houde and Chitty, 1976; Dilly et a!., 1988; Vargo and Hopkins, 1990). The OTEC sampling ofi Mobile and Tampa Bays was reported by various authors (e.g. Lawrence Berkeley Laboratory, 1980a, 1980b, 1980c; Flock and Hopkins, 1981) and summa­ rized by Commins and Horne (1979) as well as fAroo!d!95S by Iverson and Hopkins (1981). In addition to eucudeetal\976 • OTEC taxonomic and size frequency data, Cornmins .Bigpeul.I9BB *lligg.~i!i91 and Horne (1979) reported a peak in zoo­ *Bigg.~etal\997 plankton abundance in Oct. and a minimum X Zirnmmnanaru!Bigg.~J999 *WormmhetaL1000 in june 1978 at the Tampa site, whereas at the D,_ Hopkin! et !!l. "SUn&rd Station" Longitude Mobile site abundance was greatest in June Fig. 7. (A) Deepwater locations of SEAMAP and least in Aug. Approximately 98% of the plankton surveys, 1982-98. (B) Deepwater locations zooplankton were found to occur in the upper of plankton collection stations (excluding SEA­ 200 m of the water column. Diel vertical mi­ :MAP), 1958-99. gration was evident at both sites. A very extensive analysis of the zooplankton and micronekton community of the so-called mesoscale spatial variability in biomass across Standard Station in the eastern GOM (27°N, the GOM. The combined standing stock of 860W) has been done by T. L. Hopkins and zooplankton, micronekton, and ichthyoplank­ colleagues (see Hopkins et al., 1996 and ref­ ton generally varies with distance from shore erences therein). Trends in biomass and abun­ (shelf areas are generally enriched as opposed dance over depth and time at this location to the deepwater areas: Khromov, 1969a; Iver­ were elucidated in addition to the ecological son and Hopkins, 1981), depth in the water information gathered about groups of zoo­ column (highest in the upper 200 m and de­ plankton and micronekton found there. Bio­ creasing with depth: Vargo and Hopkins, mass results from these studies were not in­ 1990), and the proximity to riverine input (en­ cluded in Table 2 because they were usually riched areas downstream: Bogdanov et al., reported in dry weight units based on length­ 1968; Khromov, 1969a). Regions of upwelling, weight regressions for particular groups of or­ high current shear, or physical aggregation are ganisms rather than in bulk vVDV. However, "hot spots" that have greater standing stocks because spatial variation was not the focus of (Wormuth, 1982; Vargo and Hopkins, 1990; Hopkins' study, it is unclear whether conclu­ Lamkin, 1997; Wormuth et al., 2000). sions drawn from the data collected at this sin­ There is also evidence for temporal variabil­ gle location are generally applicable to the rest ity in deepwater stocks, both between years and of the GOM. within a given year. In general, 2-4--fold in­ Probably the most complete and systematic creases in zooplankton standing stock appear sampling of the standing stocks of zooplank­ to follow closely in time after changes in local ton, micronekton, and ichthyoplankton in the forcing factors (Bogdanov et al., 1968; Khro­ deepwater GOM is being carried out as part of mov, 1969a). These forcing factors may range an ongoing state-federal project administered from changes in river outflow (Dagg et al., by the Gulf States Marine Fisheries Cornmis­ 1991) to upwelling due to the passage of deep­ sion. Known as SEAMAP-Gulf of Mexico water eddies. Variation in overall plankton bio- (Southeast Area Monitoring and Assessment

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Program), the primary goal has been to census larvae numbers in SEAMAP bongo collections the abundance of eggs and ichthyoplankton were also elevated (Richards et al., 1989). larvae of comntercially important fish stocks. Adult tuna, as well, can be caught in such fron­ Figure 7A shows the station locations where tal zones (Roffer Offshore Fish Finding Ser­ SEAMAP cruises collected deepwater plank­ vice, pers. comm.). ton, prirnarily with 333-j-Lm mesh bongo nets Locations of other studies that produced the and 947-j-Lm mesh neuston tows according to biomass estimates listed in Table 2 are plotted standard fisheries methods but supplemented in Figure 7B. Work by Houde and Chitty with Tucker trawls on more recent cruises. (1976) and Houde et a!. (1976) included a Samples are collected one to three times per study of eastern GOM ichthyoplankton; bulk year on a 1;2 X 1/zo grid in different seasons (but plankton displacement volurrtes were reported, the majority of deepwater collections have oc­ but most of the analyses concentrated on shelf curred during April and May). Although many waters and on ichthyoplankton cmnposition of the samples collected by SEAlVIAP have been and stock estimates for species of interest rath­ from the continental shelf, so far about 2,100 er than on deepwater biomass. As in most stud­ have been tows in water depth >200 m. ies, bulk biomass was greater on the shelf than Data reports for the SEAMAP program are in the deepwater part of the study area. There produced each year and end up in the gray appeared to be a positive relationship between literature, but aliquots of the plankton collect­ ed (both sorted and unsorted) are available for bulk displacement volume and egg/larval loan. Summaries of sampling locations, bio­ abundance, although the association was not mass values, and environmental data collected always strong. Also notable is the "distinct sea­ at each plankton station are available from the sonality" in the data (especially the eggs and SEAMAP data managet~ 2 So far there has been fish larvae, due to seasonal spawning), with no summary of the interannual or decadal var­ highest biomass and numbers of eggs and lar­ iability of these data. However, some published vae during the spring-summer versus fall-win­ studies have used SEAMAP collections hom tel~ but these seasonal fluctuations were much particular regions or over certain periods of more apparent on the shelf than in the deep­ time. In 1989, Richards et al. reported that water part of the study area. both zooplankton WDV and several taxa of lar­ The studies of Biggs et al. (1988) and Biggs val fish varied across the LC boundary, being (1992) reported opportunistic sampling dur­ lower in abundance in LC interior than in the ing cruises to study LC eddies in the deepwater periphery or outside. Grimes and Finucane western GOM. The results provide further ev­ (1991) atu·ibuted increased abundance of lar­ idence that the upper 200 m of LCEs are low val fish caught in SEAMAP neuston tows taken in plankton stocks, especially in contrast to in the front between Mississippi River plume LCE periphery. With a !,4-m2 Multiple Open­ and oceanic waters to enriched primary and ing/Closing Net Environmental Sensing Sys­ secondary production there, as indicated by el­ tem (MOCNESS) (for a description of gear, evated chlorophyll a and zooplankton WDV. see Wiebe et al., 1985), Zimmerman and Biggs Recently, Lamkin (1997) used 6 yr of SEAMAP (1999) collected samples in a transit through data, 1983-88, in an investigation of the frontal a cyclone, a LCE, and the LC itself. This sam­ zones associated with the northern excursions pling documented higher standing stocks of of the LC. Lamkin found a positive correlation zooplankton and micronekton in the cyclone between the abundance oflarval nomeid fishes than in the LC or the LCE. Recently, Wormuth and the location of the northern edge of the et al. (2000) reported on extensive 1-m2 MOC­ LC. In particular, Cubiceps pauciradiatus has NESS sampling, which they supplemented with adult spawning grounds and larval habitats closely related to sharp temperature gradients. Isaacs-Kidd midwater trawl (IIQ1T) collec­ Larvae of apex predators like bluefin and yel­ tions, as a part of the GulfCet II multidisciplin­ lowfin tuna seem to be most abundant along ary study of marine mammal, sea turtle, and LC frontal zones and within eddy peripheries, seabird abundance and distribution. Their where zooplankton biomass and myctophid trawling carried out in support of this recently completed research program, which was co­ sponsored by the U.S. Geological Survey and Minerals Management Service, also document­ 2 See http:/ /www.gsmfc.org/seamap.html or write to SEAwiAP Data Manager, Southeast Fisheries Sci­ ed that cyclones had locally higher standing ence Center, Mississippi Laboratories, Bldg. 1103, stocks of zooplankton and nekton than did Rm. 218, Stennis Space Center, MS 39529. LCEs. https://aquila.usm.edu/goms/vol19/iss1/2 16 DOI: 10.18785/goms.1901.02 Biggs and Ressler: Distribution and Abundance of Phytoplankton, Zooplankton, Ichthyo BIGGS AND RESSLER-PLANKTON AND PRODUCTMTYIN DEEP WATER 23

DEEPWATER ZOOPlANKTON, lCHTHYOPLANKTON, and Smith, 1989; Asl'Uian eta!., 1994; Zhou et AND MICRONEKTON: AcoUSTIC SAMPLING a!., 1994; Griffiths and Diaz, 1996; Ressler et a!., 1998). Some of the studies cited above Besides traditional net sampling, acoustic (Ressler et a!., 1998; Wormuth et al., 2000) methods are also currently recognized as im­ have employed "sea-truth" sampling of zoo­ portant ways of studying zooplankton and mi­ plankton and micronekton with a 1-m2 mouth cronekton (Greene and Wiebe, 1990; Wiebe et area, 333-[Lm mesh size MOCNESS. With 1) in­ a!., 1997; Greene et a!., 1998). Under typical formation about the acoustical properties of open ocean conditions and with ft·equencies the ADCP and relevant hydrographic data, 2) on the order of 100 kHz, the particles respon­ net sampling of sound-scattering zooplankton sible for acoustic volume backscattering (Sv) and micronekton concurrent with ADCP sm~ are assumed to be zooplankton and micronek­ veys, and 3) some acoustic theory to refine the ton (Clay and Medwin, 1977; Stanton et a!., estimate of what is being measured and how 1994). There are several approaches to making different sizes, abundances, and taxa of zoo­ standing stock measurements of zooplankton plankton and micronekton are impacting the and micronekton with acoustics (for a survey, signal, it is possible to produce ADCP-derived see Hersey and Backus, 1962; Greene and Wie­ estimates of standing stock biomass and map be, 1990; Wiebe and Greene, 1994; Foote and zooplankton and micronekton biomass distri­ Stanton, 2000). One of the simplest is to use a butions over depth, space, and time (Figure 8). single-frequency echosounder to measure acoustic backscattering from a volume of water and to then relate this measurement to num­ DEEPWATER ZOOPLANKTON, lCHTHYOPLANKTON, ber or biomass of sound-scattering organisms AND MICRONEKTON: OPTICAL SAMPLING in that volume as determined by direct sam­ pling with nets. Near-real-time towed optical surveillance To date, there have been few acoustic sur­ with Optical Plankton Counters (OPCs) and veys of deepwater zooplankton, micronekton, Video Plankton Recorders (VPRs) offers an­ or ichthyoplankton in the GOM. Mter the ear­ other more recently developed means of sm~ ly work of Van Schuyler and Hunger (1967) veying zooplankton. Deepwater VPR studies and Thompson (1971) on acoustic volume have not been conducted in the GOM, but in backscattering, no studies with special purpose other regions VPR observations have been acoustics to measure zooplankton, micronek­ used in concert with net and acoustic sampling ton, or ichthyoplankton in the deepwater to study the coarse-scale abundance and com­ GOM have reached the published literature. position of zooplankton populations (Benfield However, both moored and vessel-mounted eta!., 1996, 1998; Davis et al., 1996). Recently, Acoustic Doppler Current Profilers (ADCPs) laser line scan imaging by dual light sheet are routinely used to measure the velocity of (DLS) technology has been developed for a near-surface currents, and recently, several vol­ towed system at the Center for Ocean Tech­ ume backcattering studies with ADCPs have been completed and published (Biggs et a!., nology at the University of South Florida 1997; Zimmerman, 1997; Ressler et a!., 1998; (USF). Known as the High Resolution Sampler Zimmerman and Biggs, 1999; Wormuth et a!., II (HRS-II), this towed system is a comprehen­ 2000). The ADCP transmits a sound pulse into sive marine particle analysis system consisting the water and then awaits the return of sound of an environmental suite of off-the-shelf in­ scattered back by passively drifting particles in struments (conductivity-temperature-depth the water column. The Doppler shift of this sensor, beam transmissometer, chlorophyll backscattered sound is then used to estimate fluorometer, irradiance sensor, and pitch and current speed and direction. However, the roll sensors); a particle analysis package con­ ADCP also measures the intensity of the back­ sisting of a commercially available OPC and scattered acoustic return, which is proportion­ the University of South Florida-designed pro­ al to the number and backscattering cross sec­ totype DLS; and a net verification system con­ tions of the particles in a given ensonified vol­ sisting of a 20-position, 162-[Lm plankton net, ume of water (Clay and Medwin, 1977; Medwin rotating cod-end carousel. Collections made and Clay, 1998). with the HRS-II system on the West Florida Although the ADCP was not designed as a continental margin have recently been report­ scientific echosounder (Brierly et a!., 1998), ed by Sutton et al., (2001), and the system en­ ADCPs have been successfully used to estimate gineering was described by Samson et al. the concentration of sound scatterers (Flagg (1999, 2000).

Published by The Aquila Digital Community, 2001 17 Gulf of Mexico Science, Vol. 19 [2001], No. 1, Art. 2 24 GULF OF MEXICO SCIENCE, 2001, VOL. 19(1)

Night, Cyclone Night, Confluence 10

42 --. 5 74 -5 0. 106 0 Q 138

170

202 2210 0010 0610 1210 1810 0010 0610 1200

Time (CDT)

-92 -87 -82 -77 -72 -67 -62

Fig. 8. False-color running plot of S,. collected with an ADCP along a north-south transect line from the deep water off the Mississippi Rive r, through a cyclone, and into a Loop Current Eddy (LCE-C) during Oct. 1996 (Davis et al., 2000: fi g . 3.14). Red and yellow areas on the p lot indicate highe r S,.; blue and purple colors indicate less intense returns. Because local time and loca tion are both changing along the x-axis, such field sun>ey data include temporal variability (highet- S,. at night than in the daytime) as well as spatial variability (higher S,. in the C)'clone than in the LCE).

DEEPWATER ZOOPLAN KTON, I CHTHYOPLANKTON , I I www-ccar. colorado. edu I ~ Ieben/ gulfmex_ AN D MICRON EKTON : A COMBINED APPROACH science/ ) now allows eddies to be tracked and FOR 21ST CENTURY SURVEYS shows they are temporally persistent though spatially variable regions of positive or negative Traditional direct sampling and alternative sea surface height. To judge "how long?" such acoustical and optical techniques are comple­ eddies need persist in order to become biolog­ mentary approaches. Net sampling provides ical hot spots, it seems to us that biologically taxonomic information that cannot currently important time scales are the lifetime of the be ga thered with acoustical or optical tech­ eddies (5-15 mo) , modified by how long it niques; it also provides necessary " sea-truth" takes for the phytoplankton to take advantage information needed to interpret acoustical and of the increased nuu·ients (clays) and how long optical data. However, acoustics and optics can this energy takes to translate to higher trophic make nearly continuous m easure ments over levels (weeks to months). Hence, we propose various temporal and spatial scales, providing that eddies that remain spun up for weeks to zooplankton-micronekton- ichthyoplankton months are temporally persistent to the pop­ data with sufficient resolution to examine tem­ ulations of organisms that inhabit them. poral and spatial trends in a manner impossi­ ble with ne t sampling at single discrete loca­ tions. This capacity is also useful give n the ACKN O WLEDGM ENTS growing amount of coarse to mesoscale ocean­ ographic data available from satelli tes. A com­ ' 'Ve g ratefully acknowl edge the support we bination of net, acousti cal , and o ptical tech­ receive d for this review from Con tinental She lf niques appeat-s to be the optimum way to study Associates, Incorporated (OCS Study MlVIS spatial and temporal " hot spots" in zooplank­ 2000-049) and additional funding from the ton and micronekton standing stock biomass, U.S. Minerals Management Service to meet and such a unification of technologies will lead publication costs. Assistance from the LATEX to better understanding of the interaction of Data Office, GulfCet Data Office, NEGOM hydrography and ecology in the deepwater Data Office, and SEAI\IIAP Data Office is also GOM. much appreciated. Gloria Guffy (Texas A&M Time-series animation of altimetry data (http: University Oceanography) and the staff at the https://aquila.usm.edu/goms/vol19/iss1/2 18 DOI: 10.18785/goms.1901.02 Biggs and Ressler: Distribution and Abundance of Phytoplankton, Zooplankton, Ichthyo BIGGS AND RESSLER-PLANKTON AND PRODUCTIVITY IN DEEP WATER 25

microtext and reference division of Texas primary productivity of the Texas-Louisiana con­ A&M University Sterling C. Evans Library tinental shelf.]. Mar. Syst. 11:237-247. helped us to locate source material for the re­ ---,D. E. SMITH, R. R. BIDIGARE, AND M.A. JOHN­ view, and Joel Ortega made the base maps with SON. 1984. In situ estimation of the population 1/12° bathymetry that we used in Figures 1, 2, density of gelatinous planktivores in Gulf of Mex­ ico surface waters. Mem. Univ. Nfdl. Occas. Pap. and 7. Comments from reviewers Frank Miiller­ Bioi. 9:17-34. Karger and a second (anonymous) reviewer ---,A. C. VASTANO, R. A. 0SSINGER, A. G. ZURITA, helped improve the readability of this review. AND A. P. FRANCO. 1988. Multidisciplinary study of warm and cold-core rings in the Gulf of Mexico. LITERATURE CITED Mem. Soc. Cienc. Nat. La Salle 48(3) :11-31. ---, R. A. ZIMMERMAN, R. GASCA, E. SUAREZ-MO­ AL-ABDULKADER, K. A. 1996. Spatial and temporal RALES, I. CAsTELLANOS, AND R. R. 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