MARINE ECOLOGY PROGRESS SERIES Published March Mar. Ecol. Prog. Ser. 23 Copepod egg production in Long Island Sound, USA, as a function of the chemical composition of seston Sigrun Huld Jonasdottir*, David Fields, Silvio Pantoja Marine Sciences Research Center. State University of New York at Stony Brook, Stony Brook. New York 11794-5000, USA ABSTRACT: The effect of the chemical composition of seston on the egg-production rates (E,: eggs female-' d-') of the copepods Acartia hudsonica Pinhey and Ternora longicornis (Muller) was studied in Long Island Sound, USA, (41" 00' N, 73' 05' W) during spring 1990. The seston was analyzed for car- bon, nitrogen, protein, carbohydrate (CHO)and fatty acid content as well as chlorophyll (chl) and cili- ate concentrations. Pnncipal component analysis (PCA) revealed significant correlations [score >0.7 on a same principal component (PC)]between in situ E, of the 2 copepod species and the chemical com- position of the seston. Protein, CH0 and specific fatty acids correlated strongly with E, of A. hudsonica (CH0 negatively). The same components were moderately correlated (score >0.6 on a same PC) with the E, of T. longicornis. The fatty acid requirements of A. hudsonica and 7.longicornis were for high o3:w6 ratios and low 20:s to 22:6 ratios. The fatty acid 22:603 was also correlated with E, of both spe- cies. The concentration of ciliates and the C:N ratio of seston did not affect E, of any of the species under any condition. Path analysis models were composed to evaluate the important mechanisms con- trolling the E, observed in this study. The analysis demonstrated the strength and importance of indi- rect relationships that were not apparent from conventional correlation statistics. The results from 4 path analysis models showed that chl and ciliates exert an important control on natural egg production rates, through their chemical composition, despite the fact that linear correlations between phyto- plankton and clliates and E, were not significant. KEY WORDS: Acartia . Ternora. Egg production . Food quality. Spring bloom . Path analysis INTRODUCTION Sound (LIS), USA, it has been observed that phyto- plankton blooms do not necessarily result in enhanced Biological and physical factors that influence the egg production of the copepods Acartia hudsonica egg-production rates (E,: eggs female-' d-l) of cope- (Weissman et al. 1993), T. longicornis and Centropages pods have been widely studied in the marine environ- typicus (W. T. Peterson pers. comm., unpubl, data).The ment. Several studies report strong correlations composition of the seston was not studied in those between the standing stock of phytoplankton, mea- investigations. sured as chlorophyll (chl) a concentration, and E, of In addition to chl concentrations, in situ egg-produc- copepods (e.g.Checkley 1980, Uye 1981, Durbin et al. tion studies to date have mainly measured correlation 1983, Runge 1985b) while others find no correlation between E, of copepods and microzooplankton con- between the 2 factors (e.g. Runge 1985a, Stearns et al. centrations (White & Roman 1992, Ki~rboe& Nielsen 1989, Durbin et al. 1992, Kleppel 1992). In Long Island 19941, carbon, nitrogen and C:N ratios (Durbin et al. 1983, Ambler 1986, Kleppel 1992, White & Roman 1992). The effects on copepod E, of other chemical 'Present address: Danish Institute for Fisheries and Marine components of seston such as fatty acids, carbohy- Research, Charlottenlund Castle, DK-2920 Charlottenlund, drates (CHO) and proteins which are also known to Denmark fluctuate with seasons and during individual blooms O Inter-Research 1995 Resale of full article not perrnjtted 88 Mar. Ecol. Prog. Ser 119: 87-98, 1995 (Morris et al. 1983, Mayzaud et al. 1989) have not been podites and adult copepods but allowed larger phyto- examined in detail before. plankton cells and chain forms to be retained in the The objective of the study presented here was to incubation water. To correct for eggs which may have define the qualitative and quantitative relationship passed through the 100 pm screening of the incubation between food and Er of 2 coastal copepod species, water, 2 to 3 controls were set up. Acartia hudsonica Pinhey and Temora longicornis Bottles were incubated for 24 h at ambient LIS tem- (Miiller) in LIS during a spring phytoplankton bloom in peratures on a 14 h dim light: l0 h dark cycle. The con- order to understand the observed lack of correlation tents were then screened using a 20 pm mesh and the between copepod egg production and phytoplankton eggs counted. The eggs of Acartia hudsonica and standing stock. Along with temperature measure- Temora longicornis were easily distinguished from ments, chl a concentration, as an indicator of phyto- each other as the eggs of A. hudsonica are translucent plankton biomass, and ciliate density, as well as the where T. longicornis eggs are dark (greenish grey). chemical composition (protein, CH0 and fatty acid The number of eggs of each species in the control (3 to content) of the seston were measured. Although this 9 eggs 1-l) were subtracted from the in situ egg-pro- study focused on the specific fatty acids (SpFA) of the duction counts. The prosome length of the females was seston but did not further separate protein and CH0 measured at the end of all incubations. into specific compounds, the importance of other spe- Sampling and measurements of seston. Concurrent cific components of the phytoplankton cells, such as with the zooplankton sampling, water was pumped amino acids and vitamins (Conklin & Provasoli 1977, from 5 and 15 m depths for various measurements of Dadd 1982, Harrison 1990),should not be ignored. seston. Temperature was measured with a mercury In this paper, the plausible interactions between food thermometer with a division of O.l°C. Duplicates of and egg production were tested by the use of path 100 m1 samples were taken for determination of phyto- analysis because the use of path analysis can yield and microzooplankton composition and were fixed insight into the interactions of variables in cases where immediately with Lugol's solution. All other water direct or indirect causal paths are Likely to be impor- samples were stored in bottles on ice in coolers until fil- tant (Li 1975). Path analysis is a form of structural Lin- tered in the laboratory, usually 2 to 3 h after sampling. ear regression analysis (Li 1975, Sokal & Rohlf 1981) The water was passed through a 100 pm screen before that uses a diagram, structured by the investigator, to all filtrations in order to remove larger animals (cope- form logical causal relationships among variables that pods, jellyfish, etc.).Samples for both total chl a (GF/F are compatible with the data (Li 1975). filters) and the >8 pm (Nuclepore) chl a fraction were This study investigates in more detail than previ- filtered (2 to 3 replicate samples each) and the concen- ously done, the relationship between the chemical trations used to estimate the standing stock of phyto- composition of particulate organic matter (POM) and plankton. Duplicate samples for carbon and nitrogen in situ E, of copepods in the marine environment. were filtered on combusted GF/F filters and dried at 60°C. Triplicate 1 1 samples for each of the protein, lipid and CH0 analyses of POM were filtered onto METHODS ignited GF/C filters. Individual filters were folded and put into 2 m1 cryogenic vials. Samples were immedi- Zooplankton sampling and egg production. Cope- ately frozen at -80°C for later chemical analysis. pods were collected with a 210 pm mesh-size plankton Ciliates were counted from a 1 m1 condensed Lugol's net from LIS off Port Jefferson, New York (41" 00' N, sample using a Sedgewick Rafter counting cell. Esti- 73" 05' W) on 5 dates (February 22, and March 2, 8, 16, mation of volume concentrations of ciliates was per- and 23) during spring 1990. The net was towed verti- formed by filtering a known volume of the Lugol's cally by hand from a depth of -25 m. The contents of the sample onto a 0.25 pm Nuclepore filter, which was put cod end were carefully emptied into a bucket filled with on a microscope slide and covered with an oil drop and ambient seawater and transported to the laboratory. Im- a cover slide. The sample was immediately analyzed mediately after arrival at the laboratory (-2 h after under an inverse compact microscope by measuring sampling), actively swimming Acartia hudsonica and >l0 of the most abundant ciliates and all of the lesser Temora longicornis females were sorted out for use in abundant ciliate types. Volumes were calculated from incubation experiments to measure Er measurements of linear dimensions using simple geo- Females were incubated in 1 1polycarbonate bottles. metric formulas corresponding to the shapes of the cil- For each copepod species, 4 to 5 bottles containing 3 to iate~. 4 females each were set up. Bottles were filled with Chemical analysis of seston. Proteins were deter- 100 pm screened ambient water from -5 m depth. mined using bicinchoninic acid (BCA) protein assay Screening of the water removed all nauplii, cope- reagent (Pierce) (Smith et al. 1985).Protein was solubi- Jonasdottir et al.. Copepod egg pr -0duction in Long Island Sound 89 lized from the particles on the filters using 1 % sodium was based on a pre-correlation analysis of E, and fatty dodecyl sulfate (lauryl)detergent prior to analysis. The acids. light extinction of the protein solution was quantified For the purpose of this study, the results of interest on a spectrophotometer at a wavelength of 562 nm. lie in the individual principal component (PC) which CH0 was measured by the 3-methyl-2-benzothia- includes egg production as a major variable.