Isotopic evidence for zooplankton as an important food source for the mysid Paramysis lacustris in the Curonian Lagoon, the South-Eastern Baltic Sea

Abstract

To evaluate the contribution of mesozooplankton to the diet of a coastal mysid Paramysis lacustris in the Curonian Lagoon, stable isotope analysis of mysids and mesozooplankton was conducted from July to October 2004. Nitrogen isotopic composition (δ¹⁵N) of both mysid bodies and dissected stomachs was analyzed and their δ¹⁵N values were compared to those in ambient mesozooplankton. We also evaluated changes in phyto- and mesozooplankton assemblages during the study period and related the community dominance structure to the isotopic composition of the mesozooplankton and mysids. Stable isotope data suggest that during the ontogenetic development, mysid diet changes to include a larger proportion of mesozooplankton and that mysids need to attain a threshold size of 8.7 ± 0.7 mm to feed on mesozooplankton. In adult mysids, δ¹⁵N values of the dissected stomachs and stomach content were indistinguishable from those of ambient mesozooplankton, indicating nearly complete reliance on mesozooplankton prey. A significant depletion of ¹⁵N in both mesozooplankton and mysids occurred during the study period, correlating with cyanobacteria abundance in the lagoon. These findings contribute to the understanding of P. lacustris' feeding biology, and the pelagic food web structure and functioning in the coastal environment.

Introduction

Mysids are generally considered to be omnivorous crustaceans feeding on phytoplankton, detritus, zooplankton and small benthic animals in lakes and estuaries (Mauchline, 1980). There are also numerous observations that the composition of mysid diet changes during their ontogeny (life-history omnivory), with herbivory dominating among juveniles and carnivory among adults (Branstrator et al., 2000, Viherluoto et al., 2000, Viherluoto and Viitasalo, 2001). The broad diet implies a variety of possible functional roles in ecosystem food webs and points to difficulties in predicting their trophic linkages and in extrapolating from one ecosystem to another. In particular, mysids are important in consumption of suspended matter in the detritus-based estuarine food webs (Fockedey and Mees, 1999), bentho-pelagic coupling by diel vertical migrations (Lasenby and Shi, 2004) and regulation of zooplankton community structure (Rudstam et al., 1989, Jane et al., 1996, Ketelaars et al., 1999, Spencer et al., 1999).

To support a commercial fishery, the Ponto-Caspian mysids of Paramysis genus were extensively introduced into many inland water reservoirs of Russia (Komarova, 1991 and the references therein) as well as of former Soviet countries, including Lithuania (Arbačiauskas, 2002). In feeding experiments conducted prior to their introduction, Paramysis lacustris showed preferential feeding on detritus and phytoplankton (Komarova, 1991) and it was therefore expected to improve the efficiency of detritus and primary production utilization in the eutrophic Curonian Lagoon. Contrary to these expectations, however, the stomach content analysis of field-collected P. lacustris has revealed a significant proportion of planktonic crustaceans, especially during nocturnal vertical migrations, suggesting that mysids feed actively on mesozooplankton (Jankauskienė, 2003).

Assessing the contribution of a particular food source to mysid diet is a difficult task, because mysids macerate their prey beyond recognition and traditional gut content analysis often fails to reliably quantify prey types in the stomach content (Mauchline, 1980). In such cases, stable isotope analysis (SIA) can provide a useful alternative and give insights into the feeding relationships between the organisms within a given food web (Minagawa and Wada, 1984, Post, 2002). This approach has been successfully applied to study trophic interactions in a variety of organisms, including mysids and zooplankton (Toda and Wada, 1990, Hansson et al., 1997, Gorokhova and Hansson, 1999, Branstrator et al., 2000, Johansson et al., 2001).

This study examined changes in nitrogen isotopic composition (δ¹⁵N) of different size classes of Paramysis lacustris and crustacean mesozooplankton in the Curonian Lagoon during the growth season. The primary objectives were to evaluate ontogenetic and seasonal variations of δ¹⁵N in mysids, and to ascertain the contribution of mesozooplankton to the diet of P. lacustris. We also evaluated the usefulness of analyzing isotopic composition of dissected mysid stomachs and compared bolus δ¹⁵N values with those of ambient mesozooplankton. When evaluating seasonal pattern in mysid and mesozooplankton δ¹⁵N, we expected to observe depletion related to the development of cyanobacteria, a phenomenon previously reported from other estuaries, including the Baltic Sea (Rolff, 2000).