We assessed metabolic balance, RNA content, and egg hatching success (EHS) in Acartia tonsa and A. clausi over a wide salinity range (2 to 33 and 16 to 33, respectively). For A. tonsa, the energy partitioning between ingestion, production and respiration was relatively constant with small differences in gross growth efficiency (GGE) and cost of growth (CG). In contrast, A. clausi exhibited significantly reduced ingestion and GGE, and highly elevated CG at salinities ≤20. In both species, RNA levels mirrored egg production. EHS was generally high in both species, but decreased by 80% for A. clausi at 16. These results contribute to the understanding of distribution patterns of both species along salinity gradients. The observed responses would allow the dominance of A. tonsa at low salinities, although its higher energetic requirement and feeding activity subject it to stronger predation pressure than competing A. clausi.

Cercopagis pengoi, a recent invader to the Baltic Sea and the Laurentian Great Lakes, is a potential competitor with fish for zooplankton prey. We used stable C and N isotope ratios to elucidate trophic relationships between C. pengoi, zooplankton (microzooplankton, 90-200 microm, mostly copepod nauplii and rotifers; mesozooplankton, >200 microm, mostly copepods), and zooplanktivorous fish (herring, size range 5-15 cm and sprat, 9-11 cm) in a coastal area of the northern Baltic Sea. The isotope ratios in C. pengoi and fish were much higher than those of zooplankton, showing general trends of enrichment with trophic level. Young-of-the-year (YOY) herring had a significantly higher (15)N/(14)N ratio than C. pengoi, suggesting of a trophic linkage between the two species. To evaluate the possible relative importance of different food sources for C. pengoi and YOY herring, two-source isotope-mixing models for N were used, with micro- and mesozooplankton as prey for C. pengoi and mesozooplankton and C. pengoi as prey for YOY herring. These models indicate that mesozooplankton was the major food source of both species. However, microzooplankton may be important prey for young stages of C. pengoi. Comparative analyses of the herring trophic position before and after the invasion by C. pengoi showed a trophic level shift from 2.6 to 3.4, indicating substantial alterations in the food web structure. Our findings contribute to a growing body of evidence, showing that C. pengoi can modify food webs and trophic interactions in invaded ecosystems.

ABSTRACT. The egg-carrying copepod Eurytemora affinis IS cles and detntus are often found in high concentrations often a characteristic and dominant component in the carbon cycles of freshwater influenced marine ecosystems in the Northern Hemisphere. In egg-carrying copepods the specific e g g produclion rate in the field can be calculated from the e g g hatching rate at in situ temperatures, the eggliemale ratio and the carbon content of the eggs and females. To determine the temperature dependent e g g hatching time (HT) of 2 populations of E. affinis, hatching experiments were conducted at 6 different temperatures (T). Hatching times ranged from 14.1 to 1.6 d at 5 and 22"C, respectively. Results from the 2 populations were pooled and fitted to a power function-HT = 187T ' 5 4 (I-' = 0.97; p < 0.001). There is a large range in the hdtch~ng t~rnes versus temperature reported In the 11tei-ature, dnd our results reveal longer hatching times than prev~ously reported To overcome this pronounced vanation in hatching tlmes, we present a significant relationship (r2 = 0.87; p < 0 001) based on all available data: HT =

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