Abstract
Zooplankton on continental shelves represent an important intermediary in the transfer of energy and matter from phytoplankton to the wider ecosystem. Their taxonomic composition and trophic interactions with phytoplankton vary in space and time, and interpreting the implications of this constantly evolving landscape remains a major challenge. Here we combine plankton taxonomic data with the analysis of biovolume spectra and stable isotopes to provide insights into the trophic interactions that occur in a shelf sea ecosystem (Celtic Sea) across the spring-summer-autumn transition. Biovolume spectra captured the seasonal development of the zooplankton community well, both in terms of total biomass and trophic positioning, and matched trophic positions estimated by stable isotope analysis. In early April, large microplankton (63–200 µm) occupied higher trophic positions than mesozooplankton (>200 µm), likely reflecting the predominance of nanoplankton (2–20 µm) that were not readily available to mesozooplankton grazers. Biomass and number of trophic levels increased during the spring bloom as elevated primary production allowed for a higher abundance of predatory species. During July, the plankton assemblage occupied relatively high trophic positions, indicating important links to the microbial loop and the recycling of organic matter. The strong correlation between biomass and community trophic level across the study suggests that the Celtic Sea is a relatively enclosed and predominantly energy-limited ecosystem. The progression of the zooplankton biomass and community structure within the central shelf region was different to that at the shelf-break, potentially reflecting increased predatory control of copepods by macrozooplankton and pelagic fishes at the shelf break. We suggest that the combination of size spectra and stable isotope techniques are highly complementary and useful for interpreting the seasonal progression of trophic interactions in the plankton.
Highlights
Shelf seas are important for ecology, fisheries and biogeochemistry (Sharples et al, this issue) and face growing pressure from anthropogenic activities as the world’s population increases (Levin et al, 2015); ∼40% of the global population lives within 100 km of the coast and depends upon shelf sea ecosystems for its wellbeing
Our estimates of zooplankton biomass and composition matched previous records well: The long-term monthly mean mesozooplankton biomass suggests higher biomass on the shelf than at the shelf break (Batten et al, 1999; Joint et al, 2001), which we see in our observations (Fig. 5; Table 2)
Our estimated biomass was approximately twice as high as the long-term monthly mean (Batten et al, 1999; Joint et al, 2001) and higher than the biomass recorded between Mar 2014 and Mar 2015 at the E1 monitoring site 40 km south of Plymouth (Djeghri et al, this issue)
Summary
Shelf seas are important for ecology, fisheries and biogeochemistry (Sharples et al, this issue) and face growing pressure from anthropogenic activities as the world’s population increases (Levin et al, 2015); ∼40% of the global population lives within 100 km of the coast and depends upon shelf sea ecosystems for its wellbeing. The development and management of shelf sea ecosystems require a holistic understanding of the factors that influence the community structures of the resident organisms and the ecosystem functions (e.g. carbon export, nutrient regeneration, secondary production, etc.) and services (e.g. food production, climate regulation) that they provide. Food production, climate regulation) that they provide This broad-scale understanding is increasingly explored and developed using ecosystem models (Hyder et al, 2015), at the shelf- (or larger) scale. Such models necessarily represent a compromise between the complexity of natural ecosystems and the need for computational efficiency.
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