Tracking seasonal changes in North Sea zooplankton trophic dynamics using stable isotopes

Trophodynamics of meso-zooplankton in the North Sea (NS) were assessed at a site in the southern NS, and at a shallow and a deep site in the central NS. Offshore and neritic species from different ecological niches, including Calanus spp., Temora spp. and Sagitta spp., were collected during seven cruises over 14 months from 2007 to 2008. Bulk stable isotope (SI) analysis, phospholipid-derived fatty acid (PLFA) compositions, and δ 13CPLFA data of meso-zooplankton and particulate organic matter (POM) were used to describe changes in zooplankton relative trophic positions (RTPs) and trophodynamics. The aim of the study was to test the hypothesis that the RTPs of zooplankton in the North Sea vary spatially and seasonally, in response to hydrographic variability, with the microbial food web playing an important role at times. Zooplankton RTPs tended to be higher during winter and lower during the phytoplankton bloom in spring. RTPs were highest for predators such as Sagitta sp. and Calanus helgolandicus and lowest for small copepods such as Pseudocalanus elongatus and zoea larvae (Brachyura). δ 15NPOM-based RTPs were only moderate surrogates for animals’ ecological niches, because of the plasticity in source materials from the herbivorous and the microbial loop food web. Common (16:0) and essential (eicosapentaenoic acid, EPA and docosahexaenoic acid, DHA) structural lipids showed relatively constant abundances. This could be explained by incorporation of PLFAs with δ 13C signatures which followed seasonal changes in bulk δ 13CPOM and PLFA δ 13CPOM signatures. This study highlighted the complementarity of three biogeochemical approaches for trophodynamic studies and substantiated conceptual views of size-based food web analysis, in which small individuals of large species may be functionally equivalent to large individuals of small species. Seasonal and spatial variability was also important in altering the relative importance of the herbivorous and microbial food webs

Temperature impact on reproduction and development of congener copepod populations

The goal of this study was to relate the temperature response of all developmental stages and reproductive biology of two congener copepod pairs inhabiting different biogeographic regions to their geographic distribution patterns. Survival of adult females and egg production, embryonic development and hatching success of the genera Centropages and Temora from two stations, in the North Sea and the Mediterranean, were studied in laboratory experiments in a temperature range from 2 to 35°C. Postembryonic development was determined from cohorts raised at temperatures between 10 and 20°C with surplus food. Tolerance limits and optima of female survival, reproduction and development distinguished the northern species C. hamatus and T. longicornis from the southern T. stylifera, while C. typicus, which is found in both regions, was intermediate. Thus, thermal preferences could in part explain distribution patterns of these species. While C. hamatus and the two Temora species showed distinct temperature ranges, C. typicus was able to tolerate different temperature conditions, resulting in its wide distribution range from the subarctic to the tropics. However, the thermal range of a species did not necessarily correlate with the optimal temperatures in the experiments. Egg production and stage development were surprisingly low in T. stylifera, which has a mere southern distribution

Seasonal cycles of egg production of two planktonic copepods Centropages typicus and Temora stylifera in the north-western Mediterranean Sea

Reproduction of the dominant copepods Centropages typicus and Temora stylifera was studied at a permanent station in the Ligurian Sea (NW Mediterranean). Seasonal patterns of egg production, clutch size, egg size and female prosome length were followed from January 1998 to December 1999. Female carbon content and weight-specific egg production were compared in autumn 1998 and spring 1999. Reproductive patterns of C. typicus and T. stylifera were very similar, indicating that reproduction was affected by the same environmental factors. Reproductive activity was highest in autumn in both species and years. A second peak of egg production was observed in early summer, which was less intense in 1999 after a bloom of salps. Egg production rates reached maximal values of 33.5 and 33.3 eggs female-1day-1 and annual means of 10.8 and 11.7 eggs female-1day-1 in Centropages and Temora, respectively. Maximal weight-specific egg production was 0.21 day-1 in both species in November 1998, when female carbon contents were 6.7 (C. typicus) and 12.0 µg C (T. stylifera). No statistical relationship between egg production and food availability or temperature was detected. Reproductive activity did not reflect the seasonal abundance patterns, with C. typicus dominating in spring and T. stylifera in autumn

Temperature impact on reproduction and development of congener marine copepods - a key to distribution patterns?

The study investigated the annual life cycles of four copepod species, thereof two congener pairs, which differentiate in their geographical and/or seasonal distribution patterns: (1) Temora longicornis and T. stylifera, (2) Centropages hamatus and C. typicus. T. longicornis occurs farthermost north and has hardly any overlap with its warm-temperate congener T. stylifera. The two Centropages species represent a homologous pair, with the cold-temperate C. hamatus and the southern-intermediate C. typicus. The distribution patterns of these species suggest that temperature is a major factor for copepod population dynamics and distribution. A field study focused mainly on the reproductive strategies of these species, which was related to temperature and phytoplankton concentration at two stations in the North Sea and the Mediterranean, respectively. In the laboratory, demographic parameters were studied in a temperature range from 2 to 35°C.Temperature plays a central role in the life cycles of small calanoid copepods. In the North Sea, T. longicornis was always present, while C. hamatus was absent from the water column in winter. Both species are known to overwinter as resting eggs in the sediment. The presence of C. typicus was irregular there at its northern distribution margin, underlying high interannual variability. The timing of maximum reproductive activity was similar between species and was recorded in spring, except in C. typicus. Significant correlations were obtained between clutch size and prosome length, which in turn was inversely correlated with temperature. Consequently, temperature controlled egg production via its effect on body size at Helgoland Roads, while food probably was not limiting. In contrast, oligotrophy governed copepod life in the Mediterranean, where reproductive rates were considerably lower, but continuous throughout the year in both T. stylifera and C. typicus. However, no statistical relationship between reproduction and food availability or temperature was observed.The laboratory experiments strengthened the assumption that temperature is one important source of interannual and regional variability. The tolerance limits and optima of survival, reproduction and development distinguished the northern species T. longicornis and C. hamatus from the southern T. stylifera, while C. typicus, which is found in both regions, was intermediate. Thus, thermal preferences can in part explain distribution patterns of copepds. C. typicus was able to tolerate different temperature conditions, resulting in its wide distribution range from the subarctic to the tropics

Development and growth rates of Centropages typicus

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Winter-spring phytoplankton blooms in Dabob Bay, Washington

Scientific investigations in Dabob Bay, Washington State, USA, have been extensive since the early 1960s, but phytoplankton blooms have been studied mostly with regard to chlorophyll concentrations and little is known about the phytoplankton species themselves. Here we provide information on the species present, their abundances during blooms, their contribution to organic carbon concentrations and the ability of some phytoplankton species to produce toxic aldehydes that may impact metazoan grazers. Multiple blooms of phytoplankton, dominated by diatoms, occurred in the late winter-early spring period, with depth-integrated chlorophyll levels ranging from <20 to 230 mg m−2 and peaks in February and April. The major bloom species included Skeletonema costatum, Thalassiosira spp. and Chaetoceros spp; Phaeocystis cf. pouchetii occurred in 2002 and 2004. Other taxa or groups of organisms that were sometimes abundant included unidentified small flagellates <10 μm in size and unidentified heterotrophic dinoflagellates. Large diatoms usually comprised most of the cell carbon, but a large, heterotrophic dinoflagellate, identified only as Gyrodinium “tear” because of its shape, was a major contributor to the microplankton carbon when present even in small numbers. Five Thalassiosira species and S. costatum were found to produce polyunsaturated aldehydes (PUA) that are known to affect copepod reproduction and hatching success. Our findings are similar to the few previous studies in the last four decades that included phytoplankton species and suggest long-term similarities and relative stability in the phytoplankton species present and their timing in Dabob Bay

Survey of the chemical defence potential of diatoms: Screening of fifty one species for alphabetagammadelta-unsaturated aldehydes

In recent years a negative influence of diatom-derived α,β,γ,δ-unsaturated aldehydes (PUA) on the reproductive success of copepods and invertebrates has been suggested. Since adverse chemical properties of diatoms would question the traditional view of the marine food web, this defense mechanism has been investigated in detail, but the PUA-release by test organisms has only been determined in a few cases. The observed effects were nevertheless frequently discussed from a general point of view often leading to contradictory conclusions. We have examined the PUA-production of 50 diatom species (71 isolates) in order to provide a basis for the interpretation of laboratory and field results on the influence of diatom food on the reproductive success of their consumers. PUA-production is species and strain dependent. Thirty-six percent of the investigated species (38% of the cultivated isolates) release α,β,γ,δ-unsaturated aldehydes upon cell disruption in concentrations from 0.01 to 9.8 fmol per cell. Thalassiosira rotula and Thalassiosira pacifica, major spring-bloom forming diatoms isolated from Roscoff (Bretagne, English Channel, France) and Puget Sound (Washington, USA) were among the PUA-producing strains