TRITON · Trophic interactions in marine plankton: copepod responses to a diatom defence strategy

Final Activity Report Summary - TRITON (Trophic interactions in marine plankton: copepod responses to a diatom defence strategy)

Interactions between different groups of the marine food web are important to understand how the marine ecosystem functions. A particularly important component is the plankton, as it serves as food source for all larger animals at sea, including fishes, birds and marine mammals. The traditional view that diatoms, a type of single-celled algae that is very numerous during spring blooms in mid-latitudes, are a high quality food for zooplankton has recently been challenged as the so called 'diatom paradox': instead of boosting the production of the primary consumers, minute planktonic crustaceans called copepods, laboratory experiments showed that copepods fed high concentrations of diatoms had significantly reduced reproductive success as compared to other diets. A field study conducted in Puget Sound (USA) showed that this effect is operant under natural conditions as well, when diatoms producing inhibitive polyunsaturated aldehydes (PUA) constitute an important fraction of the copepod's diet. This relationship may significantly shape the planktonic food web and could have major consequences for plankton dynamics and fish recruitment: if biomass production of copepods susceptible to PUA is delayed during diatom blooms, a mismatch in the timing of copepod population growth and the feeding of fish larvae would be expected. Moreover, species-specific differences in the response to algal toxins could result in a shift of taxonomic composition within the zooplankton community, giving those copepod species an advantage that can either avoid feeding on these diatoms, or are able to detoxify PUA during digestion or in their tissues. Finally, delayed population responses of grazer populations could affect the fate of algae during blooms, which could potentially sink to depth largely un-grazed, instead of being eaten and turned into zooplankton biomass in the surface. However, experiments have considered only few copepod species so far. In addition, most studies lacked knowledge on the PUA production potential of the diatoms used as food, due to missing biochemical analysis methods for PUA. In consequence, the results were ambiguous and discussed very controversially. Recently, methods have been developed to quantify PUA production of diatoms. Using results from this new technology, the present study explored if environmental stressors can stimulate the production of aldehydes in diatoms and if different important copepod grazers of the North Sea respond differently to PUA producing diatoms as food. At station L4 four polyunsaturated aldehydes were detected in natural seawater: heptadienal, octodienal, octatrienal and decadienal. Elevated concentrations, mainly of heptadienal, were recorded in spring (March to April) and mid-summer (July). While phosphate depletion in combination with very high UV radiation killed a large proportion of diatom cells in a Skeletonema marinoi culture, the remaining cells produced very high aldehyde concentrations after a recovery period of 6 days, making them more toxic to grazers than before the exposure. In contrast, Skeletonema marinoi grown under optimal conditions had no impact on adult copepod females feeding on these cells, as demonstrated by comparable survival and egg production rates when fed either S. marinoi or Rhodomonas maculate, a high quality alga used as control. Immature stages were more vulnerable, however. Survival of embryos declined after continuous feeding of their mothers on the toxic S. marinoi. There were no major differences in the responses of different copepod grazers, despite their differences in life cycles. This may indicate that no specific detoxification mechanisms have evolved, possibly because complementary food items are usually found under natural conditions, which mitigate any deleterious effects of PUA in the field.

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