Land–Sea Interactions and Consequences for Sub-Antarctic Marine Food Webs

Faeces of marine birds and mammals as substrates for microbial plankton communities

The chemical composition of the seawater soluble fraction (WSF) of yellow-legged gulls and harbour seal faeces and their impact on microbial plankton communities from an eutrophic coastal area have been tested. After characterisation of the C:N:P stoichiometry, trace metals content and organic molecular composition of the faeces, significant differences between species have been observed in all parameters. Seagull faeces present about three times larger N content than seal faeces and are also richer in trace elements except for Cu and Zn. Organic nitrogen in seagull faeces is dominated by uric acid, while the proteins are the main N source in seal faeces. It is remarkable that seagull faeces are five times more soluble in seawater than seal faeces and present a much higher N content (48.0 versus 3.5 mg N in the WSF per gram of dry faeces), >85% of which as dissolved organic nitrogen, with C:N molar ratios of 2.4 and 13 for seagull and seal faeces, respectively. Based on this contrasting N content, large differences were expected in their impact on microbial populations. To test this hypothesis, a 3-day microcosm incubation experiment was performed, in which coastal seawater was amended with realistic concentrations of the WSF of seagull or seal faeces. A significant and similar increase in bacterial biomass occurred in response to both treatments. In the case of phytoplankton, the impact of the treatment with seagull faeces was significantly larger that the effect of the treatment with seal faeces. Our data suggest that the distinct competitive abilities of phytoplankton and bacteria largely influence the potential impact of distinct animal faeces on primary productivity in coastal ecosystems. Impacts on the microbial plankton communities do not affect only this trophic level, but the whole trophic chain, contributing to nutrient recycling in coastal areas where large populations of these species are settled.

Southern Ocean Food Web Modelling: Progress, Prognoses, and Future Priorities for Research and Policy Makers

Graphical summary of multiple aspects of Southern Ocean food web structure and function including alternative energy pathways through pelagic food webs, climate change and fisheries impacts and the importance of microbial networks and benthic systems.

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Southern Ocean biological iron cycling in the pre-whaling and present ecosystems

This study aimed to create the first model of biological iron (Fe) cycling in the Southern Ocean food web. Two biomass mass-balanced Ecopath models were built to represent pre- and post-whaling ecosystem states (1900 and 2008). Functional group biomasses (tonnes wet weight km^-2) were converted to biogenic Fe pools (kg Fe km^-2) using published Fe content ranges. In both models, biogenic Fe pools and consumption in the pelagic Southern Ocean were highest for plankton and small nektonic groups. The production of plankton biomass, particularly unicellular groups, accounted for the highest annual Fe demand. Microzooplankton contributed most to biological Fe recycling, followed by carnivorous zooplankton and krill. Biological Fe recycling matched previous estimates, and, under most conditions, could entirely meet the Fe demand of bacterioplankton and phytoplankton. Iron recycling by large baleen whales was reduced 10-fold by whaling between 1900 and 2008. However, even under the 1900 scenario, the contribution of whales to biological Fe recycling was negligible compared with that of planktonic consumers. These models are a first step in examining oceanic-scale biological Fe cycling, highlighting gaps in our present knowledge and key questions for future research on the role of marine food webs in the cycling of trace elements in the sea.This article is part of the themed issue 'Biological and climatic impacts of ocean trace element chemistry'.