MARINE ECOLOGY PROGRESS SERIES Vol. 87: 275-281,1992 Published October 19 Mar. Ecol. Prog. Ser. -

Secondary production of gorgonian in the northern Gulf of Mexico

Naomi D. Mitchelll, Michael R. ~ardeau~,William W. Schroederl, Arthur C. ~enke~

Marine Science Program, The University of Alabama. PO Box 369, Dauphin Island. Alabama 36528. USA Marine Environmental Sciences Consortium. Dauphin Island Sea Lab, PO Box 369. Dauphin Island, Alabama 36528, USA Department of Biology, The University of Alabama, Box 870344, Tuscaloosa, Alabama 35487-0344, USA

ABSTRACT: Gorgonians are the most conspicuous sessile macroinvertebrates at many hard-substrate sites in the northeastern Gulf of Mexico. Colonies from 3 sites, an isolated limestone outcropping at less than 2 m depth off coastal Florida (USA) and 2 exposed shelly sandstone and sandy rnudstone carbonate areas at depths of 22 and 27 m on the inner shelf off Alabama (USA), were sampled to estimate secondary production. Maximum colony ages ranged from 5 to 10 yr. Tissue mass for each age class was estimated from determinations of coenenchyme thickness and colony surface area. Secondary production was estimated from colony densities, age distribution, biomass per age class, and the increase in colony biornass between age classes. Production estimates for hebes at the 2 offshore sites were 2.3 and 6.8 g ash-free dry mass (AFDM) yr-' while production of L. virgulata at the inshore site was 10.5 g AFDM m-2 yr-l, values similar to those reported for tropical scleractinian corals. Annual production-to-biomass ratios ranged from 0.37 to 0.45, indicating similar turnover times at all northern Gulf sites.

INTRODUCTION attractive to fishes. Gorgonian colonies also directly furnish habitat, contributing to an increased abundance Gorgonians are a conspicuous component of marine and diversity of associated organisms. For example, communities from cold-temperate to tropical climates Wendt et al. (1985) found over 1500 individuals rep- and from one to several thousand meters depth. resenting 135 on only 9 gorgonian colonies Although most species are tropical, several inhabit (3 , 3 , and 3 temperate and subtropical waters off the North Titanideum frauenfeldii) collected off the Georgia American continent (Bayer 1961). Gorgonians typically (USA) coast. Gorgonians are clearly important in these possess fan shapes and arborescent morphologies communities yet little is known about their basic biolog- which allow them to exploit relatively small amounts of ical parameters, particularly those of continental shelf hard substrate while utilizing a large volume of the species. water column (Barnes 1980). Nonetheless, availability To date, no study has addressed the contribution of of hard substrate may be a limiting factor for many gorgonians to hard bottom communities. Unlike the species (Grigg 1977, Benayahu 1985). In the north- primary producers which provide physical structure eastern Gulf of Mexico, hard substrates on the inner for most terrestrial and coastal communities, gorgoni- continental shelf are limited to relatively small areas ans are principally secondary producers (i.e. hetero- (Curray 1960, Ludwick 1964, Parker et al. 1983, trophic). An assessment of the secondary production of Schroeder et al. 1988a, b, 1989), but when present gorgonians, or the amount of tissue produced per unit they host diverse gorgonian-dominated communities space per unit time (Waters 1977, Crisp 1984), should (Schroeder et al. 1989). thus provide a first step towards understanding func- On the low relief (~0.25m) rock-rubble fields tional relationships in these systems. This study characteristic of these hardbottom features, gorgonian compares age distribution, biomass and secondary colonies provide most of the structural complexity production of 3 gorgonian populations in the sub-

O Inter-Research 1992 0171 -8630/92/0087/0275/$03.00 276 Mar. Ecol. Prog. Ser. 87: 275-281, 1992

tropical waters of the Gulf of Mexico, 2 offshore populations of Leptogorgia hebes on the inner conti- I nental shelf of Alabama (USA) and 1 inshore popula- . I ALA. tion of L. virgulata 50 m offshore in north central Florida (USA).

METHODS AND MATERIALS

Study sites. Site 1 (30'04' N, 88Q13' W) is 22 m deep and located in the direct path of typical Mobile Bay, Alabama, estuarine plumes (Abston et al. 1987, Dinnel I GULF OF MEXICO I5 v1 et al. 1990) (Fig. 1). Site 2 (30" 00' N, 87" 57' W) is 27 m deep, lies 28 km east-southeast of Site 1 and is less frequently subjected to direct influence of Mobile Bay plumes. Leptogorgia hebes is the dominant epifaunal species at these 2 sites but occasional specimens of Fig. 1. Leptogorgia hebes, L. virgulata.Study site locations of L. virgulata are present. The 2 species investigated gorgonian populations here, long considered to represent separate genera (Leptogorgia and Lophogorgia), have recently been united under Leptogorgia (Grasshoff 1988). Substrate The major tissue mass of gorgonian corals consists of at these 2 sites consists of low-relief (~0.25m) shelly individual polyps connected by coenenchyme. This sandstone slabs and sandy mudstone rubble, often tissue covers all external surfaces, surrounding an covered by a thin veneer of sand (Schroeder et al. internal skeleton (axial rod) in many species. Colony 1989). Site 1 occupies ca 0.01 km2, while Site 2 is biomass can therefore be determined by multiplying representative of a rubble field area ca 1.0 km2. external surface area times the mean coenenchyme Site 3 is near St. Theresa Beach, Florida (Fig. l),50 m thickness times a correction factor for ash-free dry from the beach, at depths of 1 to 1.5 m, in turbid waters mass (AFDM). of a small bay formed by Alligator Point. Because of its The surface area of the colonies was calculated from shallow depth, Site 3 is subject to stronger and more a known relationship between surface area of an frequent wave action (resulting in more intense sand object and its increase in weight when dipped in water scouring) and greater fluctuations in temperature and containing a surfactant (Harrod & Hall 1962, Hicks salinity than the offshore sites, as well as occasional 1977). This relationship was obtained by regressing exposure (once or twice a year during conditions of manual surface area measurements of 4 large, whole low tide combined with strong northerly winds). Lepto- colonies and 10 smaller branches of each species on gorgia virgulata is the only gorgonian species at this the weight increase after dipping. The diameter (d) site. The substrate is a low relief (< 0.25 m) limestone and height (h) of each segment of the whole specimens outcrop with scattered rock rubble estimated to cover of each species were measured manually. Similar roughly 0.02 km2 (Gotelli 1988). measurements were made on the smaller pieces. Dia- Production. Coral production was calculated from meter was measured at the midpoint of the branch or the following determinations: (1) total density of coral section. Surface areas were calculated assuming the colonies; (2) age distribution of sampled colonies; (3) shape of a cylinder (area = xdh), with minor adjust- colony biomass for each age class; and (4)increase in ments for blunt end pieces [n(d/2)']. Surface area colony biomass for each age class. measurements were then summed for each colony or Gorgonian densities were determined from 16.5 X piece. 1.8 m belt transect counts at offshore sites and from After surface area was obtained manually, each both belt transect and 0.25 m2 quadrat counts at the colony or piece was sprayed with a thin coat of varnish inshore site. Visual censuses were conducted by divers to reduce variability due to absorption, weighed to the 3 times in March at Site 1,twice each in December and nearest mg, then dipped for 10 s in water containing May at Site 2 and once each season at Site 3. Mean 2 drops of dish washing liquid per liter. Colonies and values were used in calculations. Colonies were pieces were weighed again after blotting. Surface area randomly collected along the belt transects (Slte 1 in from manual measurements was then regressed on the March 1989, Site 2 in May 1989 and Site 3 in increase in mass after dipping for each species. The November 1988) for determination of population age 2 regression equations were used to calculate the structure, biomass and colony surface area. surface area of each remaining colony. Twenty-eight bIitchell et al.: Production of gorgonians 277

Leptogorgia hebes from Site 2 and 29 L. virgulata colonies from Site 3 were dipped to determine surface area. Four colonies of each species were utilized to deter- mine mean coenenchyme thickness. Colonies were laid over a sheet of graph paper and random numbers corresponding to 1 cm squares were drawn until a sample from each of 3 regions (tips, middle and base) were selected. Coenenchyme within these squares was scraped and saved. Thickness was measured from the surface of the remaining coenenchyme to the axial rod. Thus, surface area (from dipping), multiplied by the mean coenenchyme thickness for that species Water Weight Increase (g) multiplied by a specific gravity of 1 g cm-3, resulted in an estimate of the wet weight of each colony. Fig. 2. Leptogorgia hebes, L. virgulata. Surface area re- Ash-free dry mass was determined for each species gressed on water weight increase of gorgonians after dipping in water containing a surfactant. N = 14 for each species; using the above coenenchyrne samples. Samples were smaller pieces all overlap near the origin weighed wet to the nearest 0.1 mg, dried at 100 'C for 24 h and reweighed. They were then ashed at 500 'C for 3 h, reweighed and AFDM calculated by subtract- biomass calculations. The mean coenenchyme thick- ing the ash from the dry mass. The average ratio of ness was 0.065 (* 0.007 SE) and 0.073 (* 0.006 SE) cm AFDM to wet mass for each species was multiplied by for L. hebes and L. virgulata, respectively. Surface area the wet mass of each colony to estimate AFDM. After for each colony was multiplied by mean coenenchyme biomass was determined, and the colony aged by thickness for that species and a specific gravity of counting growth rings in a cross-section of the base 1 g cm-3 to obtain wet mass per colony. Wet weight of (Mitchell et al. in press), biomass was regressed on age each colony was multiplied by the AFDM conversion to determine the mean biomass of each age class for factor for that species, 9.27 % (f 0.40 SE) and 8.94 % each species. (+ 0.23 SE) for L. hebes and L. virgulata. respectively, At Sites 2 and 3, age structure was obtained from the to obtain the bionlass per colony. same sample of colonies from which biomass was esti- Considering the relatively small sample sizes, a mated. At Site 1 it was not possible to obtain biomass regression was felt to provide better estimates of estimates from the collection analyzed for age struc- colony biomass for each age class than simply taking ture so biomass was estimated from the equation the means for each age class. Despite the obviously developed for Leptogorgia hebes from Site 2. Density great variation in biomass within each age class, (no. m-2) for each age class (D) was obtained from the regressions of biomass on age demonstrated sig- fraction of each age class times total density. nificant relationships (p < 0.01) although the r2 values Production for each age class was determined from were relatively low (Figs. 3 & 4). The biomass per age the mean density of colonies for each age class (D) multiplied by the change in colony biomass (AW g 1 Leptogorgio hebes AFDM) between each age class per colony. This repre- O I sents a version of the increment-summation method, P = ~(DAw)(Waters 1977, Benke 1984), where total production (P) is the sum of production for all age classes (Crisp 1984). Total standing stock biomass is the sum of the mean biomass values (B=L) X W)for all age classes.

RESULTS

The strong linear relationship between manual esti- mates of surface area and increased mass after dipping Fig. 3. Leptogorgja hebes. Log transformed biornass (g (r2= 0.96, p<0.01, N = 14 for Leptogorgia hebes; r2= AFDM) estimates regressed on age classes. See Tables 1 0.99, p<0.01, N = 14 for L. virgulata; Fig. 2) justified & 2 for biomass per age class estimated from this regression the use of this technique to estimate surface area for equation 278 Mar. Ecol. Prog. Ser. 87: 275-281, 1992

Table 2. Leptogorgia hebes. Secondary production estimate Leptogorgia virgulata for Site 2 y = 2.79~- 4-.62 r2= 0.44 a Age Density Biomass Production PIE (yr) (no. m-2) (g AFDM m-') (g AFDM m-2 yr-l) (yr-')

0-1 1-2 2-3 3-4 4-5 5-6 6--7 7-8 8-9 9-10 10-11 Fig. 4. Leptogorgia virgulata. Biomass (g AFDM) estimates regressed on age classes. See Table 3 for biomass per age Total class estimated from this regression equation class for Leptogorgia hebes at Sites 1 and 2 were both Table 3. Leptogorgia virguicrtd.Secondaq production estimate extrapolated from the same random sample of colonies for Site 3 collected at Site 2 since it was not possible to make a final collection at Site 1 for biornass estimates. This Age Density Biomass Production P/E step seemed reasonable as no dfferences in popula- (yr) (no. m-') (g AFDM m-2) (g AFDM m-' yr-') (~r-') tion growth characteristics or total branching com- 0-1 0.00 0.00 0.00 plexity were found between populations at Sites 1 and 1-2 0.34 0.16 0.33 2.06 2 (Mitchell et al. in press). A logarithmic transforma- 2-3 1.10 2.60 3.07 1.18 tion of biomass significantly improved the relationship 3-4 1.18 6.07 3.29 0.54 with age for L. hebes. 4-5 0.70 5.56 1.95 0.35 5-6 0.42 4.50 1.17 0.26 Colony densities determined from belt transects and 6-7 0.14 1.89 0.39 0.20 quadrat sampling were 1.6 (+ 0.26 SE) m-' at Site 1, 7-8 0.00 0.00 0.00 1.8 (k 0.40 SE) m-' at Site 2, and 4.0 (k 1.48 SE) m-2 at 8-9 0.06 1.15 0.17 0.15 Site 3. Using age distribution determined from the sac- 9-10 0.06 1.31 0.17 0.13 rificed colonies, these total densities were converted Total 4.00 23.24 10.54 0.45 into age-specific densities (Tables 1 to 3). Production of Leptogorgia hebes was 2.3 and 6.8 g AFDM m-' yr-' at Sites 1 and 2 respectively. Produc- stock biomass ratios were 0.44, 0.37, and 0.45 at Sites tion of L. virgulata was 10.5 g AFDM m-' yr-l at Site 1, 2, and 3. The reciprocal of the annual ~~Eratiois 3. Standing stock biomass of L. hebes was 5.2 and turnover time, or the time it takes to replace the bio- 18.3 g AFDM m-2 at Sites 1 and 2 respectively. Stand- mass of the population (Benke 1984). This equates to ing stock biomass of L. virgulata was 23.2 g AFDM 2.3, 2.7, and 2.2 yr for Sites 1, 2, and 3. The P/Bratio m-' at Site 3. Thus, the annual production to standing declined in general for each successive age class, as might be expected.

Table 1 Leptogorgia hebes. Secondary production estimate for Site 1 DISCUSSION

Age Density Biomass Production PIE 1 In the present study, the difference in annual (yr) (no. m-') (g AFDM m-') (g AFDM m-' yr-l) (yr-l) production estimates between Sites 1 (2.3 g AFDM 0-1 0.16 0.14 0.10 0.71 m-2 yr-') and 2 (6.8 g AFDM m-* yr-') is quite 1-2 0.27 0.44 0.24 0 54 marked and can be attributed to age structure 0 2-3 0.55 1.47 0.69 47 (Tables 1 & 2) and subsequent standing stock differ- 3-4 0 52 2,17 0.90 0.4 1 ences (Tables 1 & 2). Most gorgonian colonies at Site 4-5 0 12 0.75 0.29 0.39 5-6 0.02 0.18 0.07 0.39 1 were young and small, while a large percentage at Site 2 were older and larger (Mitchell et al. in press). Total 1.64 5.15 2.29 0.44 Leptogorgia vjrgulata at Site 3 had the highest pro- Mitchell et al . Prolduction of gorgonians 279

duction (10.5 g AFDM m-2 yr-l) of the 3 sites due to in Caribbean communities than is replaced (Vreeland high densities of young colonies (Table 3). The longer & Lasker 1989) and the array of potential predators in turnover time at Site 2 (2.70 vs 2.27 and 2.22 at Sites the northern Gulf, little evidence of grazing, in the 1 and 3) indicates older, larger colonies (higher bio- form of damaged tips, feeding scars or regenerated mass). The general decline in PIEratios with each tissue, was observed on the colonies we examined. successive age class (Tables 1 to 3) is a function of Mucus has long been viewed as a source of energy larger, older individuals contributing much to popula- transfer in coral reef systems (Johannes 1967, Qasim & tion biomass but a lower fraction to production Sankaranarayanan 1970, Benson & Muscatine 1974, (Robertson 1979). Coffroth 1990). Mucus sheet production by the gorgon- Most coral production studies are energy flow esti- ian Briarium asbestlnum (: ) mates based on oxygen measurements (Odum & from the Miami, Florida region was approximately Odum 1955, Kantvisher & Wainwright 1967, Davies 1 6 g dry mass m-' yr-' (calculated from Rublee et al. 1980, Lewis & Post 1982). If our AFDM estimates are 1980). Leptogorgia hebesin the northern gulf also pro- converted to energy values (g AFDM X 5 = kcal; Crisp duced mucus but we were unable to document regular 1975), we obtain 12, 34, and 53 kcal m-' yr-l at Sites 1, or predictable intervals. Coffroth (1988) suggested that 2, and 3 respectively. These figures are similar to the because of the low energy value and episodic nature of scleractinian coral production estimates reported by mucus sheets, they do not play a substantial role in reef Lewis (1981),which ranged from 29 to 57 kcal m-2 yr-' food webs. for 5 of the 6 species; the sixth was estimated at Our analyses represent an approach to estimating 783 kcal m-2 yr-l. Production estimates of various production of gorgonian corals based upon annual marine benthic populations presented by both War- biomass accumulation. This first attempt to measure wick (1980) and Lewis (1981) exhibited a wide range of productivity by a dominant species on deep temper- values, although most were below 50 kcal m-' yr.'. ate reefs suffers from small samples sizes and large Ash-free dry mass comprises a relatively low per- vanances, common deficiencies when dealing with centage of total coenenchyme dry mass due to the populations that are difficult to sample. Our values, presence of calcareous spicules embedded in the however, represent an initial estimate of the contri- tissue. AFDM percentages were 8.9 for Leptogorgia bution of an important fauna1 component of an off- hebes and 9.3 for L. vlrgulata. These fall within the shore habitat that provides cover and feeding areas range (9 to 24) of 5 species of tropical gorgonians for many fishes and invertebrates (Cummins et al. reported by Lewis & Post (1982),but were lower than 1962, Buchanan 1973, Bright et al. 1981). While not values (16 to 71) presented by Vreeland & Lasker explicitly accounting for non-lethal and (1989). mucus production, it provides a base value for pro- The relative importance of autotrophic versus duction that probably represents a high fraction of heterotrophic nutrition in gorgonians is unclear the total. (Muscatine 1973), since endosymbiotic zooxanthellae may contribute some fraction of biomass accumula- tion by primary prod.uction. Although Leptogorgia setacea has been reported to have zooxanthellae Acknowledgements. This project was supported in part by (Ciereszko 1962), some species remain asymbiotic, NOAA Office of Sea Grant, Department of Commerce. under Grant Nos. NA85AA-D-SG005, and NA89AA-D- particularly those found at greater depths (Goldberg SG016 (Projects R/ER-19-PD, R/ER-19, and E/O-l6), the 1973, Kinzie 1973). Since our populations are found at M~ssissippi-Alabama Sea Grant Consortium, NOAA's low light levels 111.5 (f 0.6 SE) pE S-' m-2 at Site 2 in Nat~onal Undersea Research Center at the University of winter] at all 3 northern Gulf study sites due to depth, North Carolina at M1ilmington under Grant No. NA88AA-D- UR004, The University of Alabama and the Marine Environ- turbidity or both, autotrophic nutrition probably plays mental Sciences Consortium. Fellowship support for N.D.M. a relatively minor role in production under these was received from the Mississippi-Alabama Sea Grant conditions. Consortium, the Patr~ciaRoberts-Harris Foundation through Production estimates reported in this study are likely The University of Alabama, and the Mobil and Shell Oil to be conservative because they include only biomass Company Foundations through the Marine Environmental Sciences Consortium. F. M. Bayer courteously verif~ed the gained from year to year and do not measure pro- gorgonian species in this study. Thanks also go to everyone duction lost to nonlethal predation or as mucus. who aided in field collections especially to J Dindo for Gorgonians have parasites and predators, including making numerous offshore dives possible, and to C. Wood the snails Cyphoma gibbosumand Simnialena uni- for her typing assistance. Rev~ekvby J. Valentine substan- tially improved the manuscript Th~srepresents Contribu- plicata, a polychaete Hermodice carunculata and the tion Number 192 of the Marlne Environmental Sciences Atlantic spadefish Chaetodipterus faber. However, Consortium and Contribution Number 190 from the Univer- despite reports that more tissue may be lost to grazing sity of Alabama Aquatic Biology Program. 280 Mar. Ecol. Prog. Ser.

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This article was submitted to the edjtor Manuscript first recejved: May 12, 1992 Revised version accepted: August 28, 1992