BULLETIN OF MARINE SCIENCE, 37(2): 726-738, 1985

ABUNDANCE, AGE STRUCTURE AND IN SITU EGG PRODUCTION RATES OF THE LONGICORNIS IN LONG ISLAND SOUND, NEW YORK

William T. Peterson

ABSTRACT The population dynamics of Temora longicornis in Long Island Sound, New York is being studied to determine the relative degree to which this species is food-limited vs. predator- limited. The plankton in the Sound has been sampled at weekly intervals from March-July 1982 and 1983. Ofthe three boreal present at this time (Temora longicornis, Acartia hudsonica and Pseui/ocalanus spp.), Temora is the dominant species. Temora eggs were most abundant within the upper 5 m and each successive life cycle stage was found deeper in the water column. Adults live at and near the sediment-water interface (Z = 35 m). Population age structure is characterized by distinct cohorts produced during discrete egg laying events. Fecundity was highest during the mid-winter bloom in February and March (30-40 eggs per female per day), intermediate during modest blooms in May and June (5-15 eggs per day) but nil during most of the February-July growth season, when standing crops were relatively low, 2 J.Lgchlorophyll per liter. Females were short-lived, surviving only 3 days on average. Female death rates may be high because the most abundant planktivorous fish in Long Island Sound (, Ammodytes americanus) feeds almost exclusively on adult Temora longicornis. by sand lance may have little effect on Temora popu- lation dynamics because Temora are most fecund in February-March, a time when predation by sand lance is relatively low. The population seems to be controlled by food-limitation of egg production.

A basic principle of population ecology is that all plant and populations have the potential for exponential growth, yet under most natural conditions, maximum population growth rates are seldom achieved. A key challenge to ecol- ogists is to elucidate the causal factors which limit population growth. One way to investigate limiting factors is to follow the physiological ecology approach, i.e., by studying the effects of vaJious environmental factors on life history parameters through controlled laboratory studies. An example here would be the work of Vidal (1980) on the effects of food concentration and temperature on the devel- opment and growth of Pseudocalanus spp. and Calanus pacificus. Another ap- proach is to sample natural populations at frequent intervals. The purpose of such studies would be to relate observed changes in abundance and age structure of a population in time, with various environmental variables, in order to gain a better idea of which environmental factors might control population size. Once suspected key factors have been identified, one can then plan laboratory experiments which focus on those processes which seem to control secondary production and pop- ulation dynamics. Examples of this approach would be the studies of the life history and population dynamics of various Acartia species by Landry (1978), Durbin and Durbin (1981), Johnson (1981), Uye (1982), and Durbin et al. (1983), and of Calanus marshallae by Peterson (1980). This paper describes the distribution, abundance, age structure, and in situ rates offecundity of the copepod Temora longicornis in Long Island Sound. This species is a dominant member of the spring and early summer copepod assemblage in the Sound. The numerical dominant is A. hudsonica, but on a biomass basis, T.

726 PETERSON: POPULATION DYNAMICS OF TEMORA 727 longicornis is first ranked, reaching a peak of 500 ~g dry weight litecl in June, greater than Acartia by a factor ranging from two to five. The third-ranked species is Pseudocalanus spp. This is in striking contrast to nearby Narragansett Bay where A. hudsonica is both the numerical and biomass dominant, achieving a biomass of 500 ~g liter-l (Durbin and Durbin, 1981). Both T. longicornis and Pseudocalanus are minor constituents in that ecosystem (Martin, 1965). The boreal copepod assemblage in Long Island Sound is also different from the adjacent Block Island Sound, where Centropages and Pseudocalanus are the dominant forms, Temora is third ranked, and Acartia hudsonica is uncommon (Deevey, 1952). These observations suggest that Long Island Sound is a relatively closed system. METHODS

The data described below were collected at a shore station and during cruises at weekly intervals from March-July 1982 and 1983 at two open-water stations in Long Island Sound. Station locations are shown in Figure 1. At each station, temperature, salinity and conductivity were measured with the aid of a Beckman RS-5 Induction Salinometer. Water was pumped from discrete depths of I, 3, 5,10, 15,20,30 and 37 m (water depth permitting) with a plankton pump (Jabsco "Water Puppy"; flow rate 12 liter min-') and 1.9-cm-i.d. hose. From the pump stream, 100 ml of water was filtered through GF/C filters for later analysis of chlorophyll, and the filtrate was saved for later analysis of nitrate concentration. Six liters of water were filtered through a 64-~m Nitex screen for later enu- meration of copepod life cycle stages. The was also sampled with a 0.5-m diameter, 202-~m mesh net, hauled vertically through the entire water column. In the laboratory, chlorophyll was analyzed by standard methods (Strickland and Parsons, 1972). For the zooplankton pump samples, the entire sample was counted, yielding a count of 100 to 800 per sample. For the plankton net samples, duplicate 1.1-ml piston-pipette subsamples were withdrawn and counted. Copepods were identified to life cycle stage (egg, nauplius I-V pooled, all other stages enumerated separately). The data on egg abundance were converted to in situ rates of egg production using the egg-ratio method: F=E/AxD where F is fecundity in units of eggs female-' day-I, E is egg abundance in number liter-', A is female abundance in number liter-I, and D is the egg development time at the observed in situ water temperature.

RESULTS Environmental Variables. - The seasonal changes in temperature, salinity, nitrate, chlorophyll, and abundances of the dominant copepod species for 1982 are shown in Figure 2. The data were taken at our shore station. Temperature ranged from less than zero (in February) to 24°C in August, and salinity from 240/00in spring to 290/00during winter. Nitrate concentrations ranged from 0.1 ~m (spring and summer) to 20 ~m (winter). The phytoplankton growth season extended from February through November. Highest chlorophyll concentrations were found dur- 1 ing the mid-winter phytoplankton bloom, 25 ~g chlorophyll liter- , in February. Lesser blooms occurred regularly in late-spring and summer triggered by spring tide-new moon tidal mixing events. Blooms can also be initiated aperiodically when grazing stress is reduced, especially in August during the summer ctenophore invasion. Modest fall blooms occurred during September, October and November. Seasonal Cycles in Copepod Species Composition. - Two distinct copepod assem- blages occurred during the calendar year. The dominant species during spring months were Acartia hudsonica, Pseudocalanus spp. and Temora longicornis. During summer and autumn months, these boreal species completely disappeared and were replaced by Acartia tonsa, Oithona simi/is and Paracalanus crassirostris. This seasonal replacement of species began in July when surface and bottom temperatures were 20 and 17°C, respectively. Curiously, the summer species per- 728 BULLETIN OF MARINE SCIENCE, VOL. 37, NO.2, 1985

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Figure 2. Seasonal changes in temperature, salinity, nitrate, chlorophyll and copepod species com- position at the shore station. Temperature and salinity at the offshore stations 2 and 4 are indicated by the squares and triangles, respectively. 730 BULLETIN OF MARINE SCIENCE, VOL. 37, NO.2, 1985

MARCH APRil MAY JUNE JULY MARCH APRIL MAY JurlE JULY ;:;\'·1'~~ 24 9 14 20 21 5 12 19 26 2 9 16 23 30 7 14 e:i\,L_ '0

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MARCH APRIL MAY JUNE JULY MARCH APRIL MAY JUNE JULY 24 '314202751219262'3 162330 714 24 9 14 20 27 5 2. 2 fB 23 30 1 " " • "

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Figure 3. Distribution and ablmdance of chlorophyll (Ilg per liter) and Temora longicornis life cycle stages (No. per liter) at Station 4, during March-July 1982.

sisted in the plankton during all months of the year, so, during the winter months and at the beginning of the spring bloom, all six of the ecologically dominant copepod species were present in the Sound. Only two of them (Temora longicornis and Acartia hudsonica) began to increase in numbers with the initiation of the bloom. This suggests that even though the summer species managed to survive temperatures ranging from below zero to 25°C, they may not be capable of pro- ducing eggs at cool temperatures. Abundance and Vertical Distribution. - The vertical distribution of chlorophyll, Temora longicornis eggs, nauplii, copepodites 1-3, copepodites 4-5 and adults is shown as a series of time-depth snapshots in Figure 3 (for the 1982 cruises) and Figure 4 (for the 1983 cruises). The data are from our offshore Station 4. Distinct patterns were seen in the chlorophyll distributions. Chlorophyll was uniformly distributed with depth during March-April, months when the Sound was well- mixed. With the onset of seasonal stratification in April (1983) and May (1982), chlorophyll was abundant only within the upper 5-10 m-thick wind-mixed layer. Eggs were most abundant at depths of 3-5 m. Nauplii were found deeper in the PETERSON: POPULATION DYNAMICS OF TEMORA 731

JAN F£B 12 26

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Figure 4. Distribution and abundance of chlorophyll (/Lgper liter) and Temora longicornis life cycle stages (No. per liter) at Station 4, during March-July 1983.

water column than eggs, being most abundant between depths of 3-15 m. Co- pepodites moved deeper in the water column, with maximum abundances gen- erally occurring between 10 and 20 m. Adults were most abundant either on the floor of the Sound or within the lower 10 m of the water column. On one occasion (2 June 1982 at Station 2 [see Fig. 1]), 300 adults litec1 were found in a sample collected with the pump-hose from the sediment-water interface. Population Retention Mechanism. - The observed vertical separation of stages with ontogeny probably serves to maintain the population within Long Island Sound. In this estuary, seaward flow is confined to the upper 10-15 m of the water column, up-estuary flow to the lower 20 rn. Eggs and nauplii would be carried in a seaward direction but the older stages would be returned because of the tendency for copepodite stages to move deeper in the water column. However, 732 BULLETIN OF MARINE SCIENCE, VOL. 37, NO.2, 1985

10,000

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1624 7 II 21 29 5 1924 I 81320 27 5" 1826 MAR APR MAY JUN JUL 1982 1983

Figure S. Cohort development during 1982 (left) and 1983 (right) at Station 4, Three cohorts were initiated during each year. PETERSON: POPULATION DYNAMICS OF TEMORA 733 even though the observed separation of stages with depth could serve to retain the population within a deep estuary like Long Island Sound, population retention is a trivial problem because in the Sound net residual flow is very small, only a few cm sec-l (Firstenberg, 1982). Age Structure and Cohort Development. -A more detailed illustration of the tem- poral changes in the abundance of each life cycle stage is shown in Figure 5. In 1982, a total of three cohorts may have been produced. Two may have resulted from the mid-winter bloom; the first by eggs produced from March through 9 April, and the second by eggs produced from 9 April through 5 May. Evidence for the existence of the first cohort is only clearly seen in the abundance data for stages Nauplius VI through C4, however. For the second cohort, development from egg to adult was seen. The third cohort in 1982, initiated on 18 June, was only seen through Nauplius VI. During 1983, three cohorts were again produced, but only the first could be followed unambiguously from egg to adult. Generation times were estimated to be 59 days for the first cohort (24 March-19 May). This compared to 45 days for the second cohort of 1982. For the second and third cohorts in 1983, generation times were on the order of 40 days (13 May-24 June) and 32 days (6 June-8 July). Survivorship from eggto fifth copepodite stage could be calculated for the 1982 cohort II, and for the 1983 cohorts I, II, and III. Values were 36%, 8%, 15% and 0.6%, respectively. Fecundity. -Cohorts were initiated by bursts in eggproduction. During both years of this study, the mid-winter bloom in February/March led to the first burst in egg production by female Temora longicornis. Maximum egg abundances coin- cided with peak concentrations of chlorophyll, and eggproduction lasted for about 30 days (Fig. 6, upper panel). After the bloom, fecundity of Temora began to become limited by phytoplankton abundance. The primary post-bloom food source may have been material from the bloom which had settled to the floor of the Sound. Female Temora could have been feeding in bottom waters by day then migrating to the surface by night where egg laying takes place. During spring 1982 especially, chlorophyll concentrations at the floor of the Sound (Z = 35 m) were two to five times greater than concentrations within the upper 5 m of the water column. If this scenario of Temora feeding mostly in bottom waters following the bloom is correct, it would suggest that a greater proportion of the mid-winter bloom is channeled into the planktonic system than we had previously thought. Other workers have noted that a large proportion of the "spring" bloom in coastal environments does not enter a pelagic food web but rather sediments out of the water column (Smetacek et al., 1978; Walsh, 1983). The suggestion has been made that most of the bloom enters a benthic food web but this may not be the case for Long Island Sound. Once this residual plant biomass had been grazed down to 2 ILgchlorophyll liter-I, Temora egg production ceased all together (Fig. 6). Egg production did not resume until the appearance of phytoplankton blooms in May, June and July. None of the increases in fecundity seen in late-spring rivaled the egg production during the mid-winter bloom, however. During most of the February-July growth season when chlorophyll concentrations ranged from 2-81Lgliter-I, Temora either did not produce eggs or produced them at rates far lower than rates observed during the mid-winter bloom.

Female Longevity and Predation. - The mean life span of females was calculated by the method of Green (1976), a technique also used by Uye (1982). See those papers for details. For Temora, females were found to be very short-lived, sur- 734 BULLETIN OF MARINE SCIENCE, VOL. 37, NO.2, 1985

15 • CHLOROPHYLL

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1982 1983

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Figure 6. Relationships between chlorophyll and fecundity of Temora /ongicornis. (Upper) Time series of chlorophyll and in situ egg production rates for 1982 and 1983. Circles are chlorophyll averaged over the upper 5 m of the water column, stars, the lower 5 m. After May 1982 and April 1983, chlorophyll in bottom waters of the Sound did not exceed I Ilg per liter so data are not shown. (Lower) Scatter diagram of fecundity vs, chlorophyll. Triangles are eggs vs. chlorophyll within the upper 5 m of the water column during 1982 and 1983, hexagons are 1981 data. The circles are eggs within the upper 5 m of the water column vs. chlorophyll within the lower 5 m of the water column for those dates in 1982 when bottom water chlorophyll concentrations were greater than surface concentrations. viving on average about 3.1 days. Longevity decreased linearly from approxi- mately 10 days in March to less than 1 day in late May. Longevities increased to about 10 days in late June then went to a in late July when water temperatures exceeded the lethal limit for Temora (20DC). These observed life spans in the field are considerably lower than laboratory measurements of 50 days at 7DC (Peterson, unpubl.) and 45 days at 12DC (Harris and Paffenhofer, 1976).

DISCUSSION The ecological dominance of Temora longicornis in Long Island Sound is some- what of a mystery. It is present but is insignificant in nearby bays and estuaries PETERSON: POPULATION DYNAMICS OF TEMORA 735

(Narragansett Bay, Rhode Island, Durbin and Durbin, 1981; Peconic Bay at the eastern end of Long Island, Turner, 1982; lower Hudson River estuary, Stepien et a1., 1981; Raritan Bay, New Jersey, Jeffries, 1962). The ecological dominant during spring and early summer in these systems is Acartia hudsonica (formerly known as Acartia clausi). In open coastal waters, Temora longicornis is also a sub-dominant, ranking third behind Centropages typicus and Pseudocalanus in both Block Island Sound (Deevey, 1952) and the New York Bight (Judkins et a1., 1980). At more distant geographical locations, T. longicornis is also a sub-dom- inant, being third, fourth or fifth ranked in Bedford Basin, Nova Scotia (Poulet, 1978), English Channel off Plymouth (Digby, 1950), Loch Striven, Scotland (Mar- shall, 1949), North Sea off Northumberland (Evans, 1973) and off the coast of Sweden (Eriksson, 1973). Temperature and salinity are not important factors controlling the population because the annual cycle ofthese two variables is the same in Long Island Sound as in Narragansett Bay and Peconic Bay. The relatively deep water column in Long Island Sound (Z = 30-40 m) compared to the shallower (< 10m) estuarine systems listed above may be important for population retention. Temora eggs are buoyant, so they would "wash-out" of a shallow estuary. In contrast, the eggs of Acartia sink and hatch out of the sediments, a strategy that must help retain Acartia populations in estuaries. The interaction between the distinct two-layer flow in Long Island Sound and the pronounced vertical separation of stages with depth must aid in the retention of Temora within the Sound, and thus contribute to the population achieving rather high numbers. The age structure of the Temora longicornis population was characterized by cohorts produced by prolonged (15-30 day) bursts in egg production. Increases in fecundity occurred during phytoplankton blooms. The data shown in Figure 6 (lower panel) show that egg laying rates reached a maximum only at very high chlorophyll concentrations, and were nil at chlorophyll concentrations ranging from 2-5 JLgliter-I. This result is somewhat troubling because Harris and Paf· fenhofer (1976) have shown that in the laboratory, the amount of phytoplankton represented by only 2 JLgchlorophyll (=100 JLgcarbon liter-I) was more than sufficient to produce maximum ingestion and growth rates by this copepod. This suggests that in Long Island Sound, total chlorophyll may be a poor indicator of the quantity of food available to copepods. We hypothesize that T. longicornis fecundity is food-limited, being controlled by phytoplankton size (i.e., the cells are too small to be ingested), a result shown for this animal in Long Island Sound in June 1978 by O'Connors et a1. (1980). The in situ egg production rates dropped to zero in early May (1982) and mid- April (1983), a time coincident with the onset of seasonal stratification. It is possible that egg production dropped because of changes in phytoplankton taxo- nomic composition from to small flagellates. It is frequently observed that shifts in phytoplankton species composition are associated with changes in stratification: mixed water columns are dominated by large diatoms, stratified water columns by small, motile taxa (e.g., see Grice et a1., 1981; Oviatt, 1981). Associated with this hypothesized shift in species composition and size, the ver- tical distribution of chlorophyll changed markedly from being uniformly distrib- uted with depth (before stratification) to being abundant only within the upper 5 m of the water column (after stratification). The effects of size of phytoplankter have been implicated in two studies of in situ eggproduction. Both CheckIey (1980b) and Ambler (1982) found that variabil- ity in the in situ rates of egg production by Paracalanus parvus and Acartia tonsa, respectively, was better explained by comparing fecundity to the > 5-JLm size 736 BULLETIN OF MARINE SCIENCE, VOL. 37, NO.2, 1985 fraction of chlorophyll rather than to total chlorophyll. Both of these copepods (in fact most copepods other than Neocalanus cristatus and Pseudocalanus) prob- ably cannot remove particles smaller than 5 ILm from the water with any great efficiency because intersetular distances on setules of the second maxilla are greater than 5 ILm (Nival and Nival, 1976; Ninivaggi, 1979). The possible interaction between phytoplankton size and food quality is seen in the in situ egg production rates reported for Acartia clausii, A. tonsa and Temora longicornis by Landry (1978), Durbin et al. (1983) and Peterson (this issue). Landry found that in situ egg production rates for A. clausii approached a max- imum of 15 eggs day-l only at chlorophyll concentrations in excess of 22 ILgliter- 1• Durbin et al. (1983) reported that maximum egg laying rates of 40 eggs female-I day-I for A. tonsa were only achieved at chlorophyll concentrations of20 ILgliter-1 and higher. In contrast, studies by Uye (1981) on A. clausi and A. steurii, Valentin (1972) on A. clausii, and CheckIey (1980a) on Paracalanus parvus showed that maximum egg production rates in the field occurred at chlorophyll concentrations comparable to laboratory-measured rates (i.e., at 2 ILgchlorophyll liter-I). All of these results suggest that the food-environment of copepods cannot always be adequately described by a simple measure of phytoplankton biomass such as total chlorophyll. The final topic to be discussed is the implications of the observation that female Temora longicornis are short-lived, surviving only 3 days on average. Similar results have been obtained in other studies of population dynamics of copepods in coastal systems (Landry, 1978; Peterson, 1980; Johnson, 1981; Uye, 1982), and all have suggested that size-selective predation by planktivorous fish is re- sponsible for reduced life expectancy offemale copepods. It is possible that female death rates are high because the most abundant planktivorous fish in Long Island Sound, the American sand lance (Ammodytes americanus), feeds almost exclu- sively on adult Temora during April and May (McKown, 1984). It is beyond the scope of this paper to discuss the details of our work on zooplankton and sand lance interactions although a brief summary follows: during April and May, abun- dances of juvenile sand lance ranged from 0.3 to 3 fish m-2 (McKown, 1984; beach seine data). In the laboratory, we determined that juvenile sand lance consume 2,000-3,000 adult Temora longicornis per day. From our estimates of sand lance abundance and feeding rates, and of T. longicornis abundances, we calculated that sand lance may consume between 10 and 30% of the available Temora per day (McKown and Peterson, unpubl.). Sand lance may not be a major predator, however, because they only reside in and near sandy substrates. In Long Island Sound, sandy sediments are confined to the shoreline out to a depth of 5- 10m. In deeper waters, sandy sediments are found only near the eastern end of the Sound. Another predator which may consume significant amounts of Temora could be a benthic invertebrate. Recent studies on the feeding behavior of burrowing sea anemones (Cerianthus) suggest that they may be important because they consume adult Temora (B. Sullivan, University of Rhode Island, pers. comm.). These organisms live in silty substrates which predominate throughout most of Long Island Sound. Since Temora live at and near the sediment-water interface, they may be vulnerable to sea anemone predation. In conclusion, our present thinking is that although predation by sand lance and sea anemones on adult Temora longicornis may be heavy, this may have little direct effect on copepod population dynamics because female Temora are most fecund in March, a time when adult sand lance are not present in the Sound, and when sea anemone feeding rates are probably low (due to low temperatures). PETERSON: POPULATION DYNAMICS OF TEMORA 737

It is hypothesized that egg production (and hence population growth) is limited by phytoplankton abundance (Fig. 6), not by a shortage offemales (Figs. 3 and 4).

LITERATURE CITED

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DATEACCEPTED: January 28, 1985.

ADDRESS: Marine Sciences Research Center, SUNY-Stony Brook, New York 11794. Contribution No. 488, Marine Science Research Center.