D. van Oevelen

Surviving in a Marine Desert: The Sponge Loop Retains Resources Within Coral Reefs

Sponge Pump “Darwins Paradox” asks how productive and diverse ecosystems like coral reefs thrive in the marine equivalent of a desert. De Goeij et al. (p. 108) now show that coral reef sponges are part of a highly efficient recycling pathway for dissolved organic matter (DOM), converting it, via rapid sponge-cell turnover, into cellular detritus that becomes food for reef consumers. DOM transfer through the sponge loop approaches the gross primary production rates required for the entire coral reef ecosystem. Sponges take up dissolved organic matter and convert it into consumable cellular material. Ever since Darwin’s early descriptions of coral reefs, scientists have debated how one of the world’s most productive and diverse ecosystems can thrive in the marine equivalent of a desert. It is an enigma how the flux of dissolved organic matter (DOM), the largest resource produced on reefs, is transferred to higher trophic levels. Here we show that sponges make DOM available to fauna by rapidly expelling filter cells as detritus that is subsequently consumed by reef fauna. This “sponge loop” was confirmed in aquarium and in situ food web experiments, using 13C- and 15N-enriched DOM. The DOM-sponge-fauna pathway explains why biological hot spots such as coral reefs persist in oligotrophic seas—the reef’s paradox—and has implications for reef ecosystem functioning and conservation strategies.

Carbon flows through a benthic food web: Integrating biomass, isotope and tracer data.

The herbivorous, detrital and microbial pathways are major components of marine food webs. Although it is commonly recognized that these pathways can be linked in several ways, elucidating carbon transfers between or within these pathways remains a challenge. Intertidal flat communities are driven by a wide spectrum of organic matter sources that support these different pathways within the food web. Here we reconstruct carbon pathways using inverse analysis based on mass balancing, stable isotope signatures and tracer data. Data were available on biomass, total carbon production and processing, integrated diet information from stable isotope signatures and the transfer of recently produced carbon through the food web from an isotope tracer study. The integration of these data improved the quality of the inverse food web reconstruction considerably, as demonstrated explicitly by an uncertainty analysis. Deposition of detritus (detrital pathway) from the water column and subsequent assimilation and respiration by bacteria and to a lesser extent by microbenthos (microbial pathway) dominated the food web. Secondary production was dominated by bacteria (600 mg C m −2 d −1 ), but transfer to higher trophic levels was limited to 9% and most bacterial carbon was recycled back to dissolved organic carbon (DOC) and detritus. Microbenthos secondary production (77 mg C m −2 d −1 ) was supported by DOC (73%) and detritus (26%) and was entirely transferred up the food web. The higher trophic levels consisting of nematodes, meiobenthos (copepods, ostracods and foraminifera) and macrobenthos fed highly selectively and relied primarily on microphytobenthos and pelagic primary production (herbivorous pathway). Deposit feeding is a common feeding mode among these sediment dwelling fauna, but detritivory was negligible due to this selective feeding. This strong resource selectivity implies that the herbivorous and detrital-microbial pathways function more or less autonomously, with limited interaction.

Uncertainties in ecological, chemical and physiological parameters of a bioaccumulation model: implications for internal concentrations and tissue based risk quotients.

Bioaccumulation models predict internal contaminant concentrations (c(i)) using ecological, chemical and physiological parameters. Here we analyse the effect of uncertainties on these parameters on bioaccumulation model predictions. Simultaneously considering the uncertainties on all these parameters in a bioaccumulation model resulted in uncertainty ranges of c(i) that increased with the octanol water partition coefficient K(ow) and reached maxima of up to 1.25 log units for mesozooplankton and up to 1.45 log units fish at logK(ow)=8. A global sensitivity analysis (SA) was performed to rank the contribution of different parameters to the observed uncertainty. The SA demonstrated that this interspecies difference resulted predominantly from uncertain production rates of fish. The K(ow), the water concentration and organic carbon-octanol proportionality constant were important drivers of uncertainty on c(i) for both species. A tissue based risk quotient (RQ(tissue)) combining uncertainty on c(i) with realistic tissue based effect thresholds indicated that fish were up to 10 times more probable to have RQ(tissue)>1 than mesozooplankton, depending on the considered threshold value. Conventional exposure based risk quotients were up to 5 times less probable to exceed one than were corresponding RQ(tissue), and this for both species.