Krill vs salps: dominance shift from krill to salps is associated with higher dissolved N:P ratios

Christoph Plum, Philipp M. Wenta, Dominik Bahlburg and Stefanie Devi Moorthi, University of Oldenburg, Institute for Chemistry and Biology of the Marine Environment, Wilhelmshaven, Germany
Changes in climate conditions and shifts in phytoplankton composition towards small-sized cells along the West Antarctic Peninsula (WAP) have resulted in decreasing krill but increasing salp populations. Krill and salps, mainly Salpa thompsoni, represent important macrozooplankton grazers at the WAP, but differ profoundly in their feeding biology, population dynamics, excretion rates and stoichiometric demands. Changes in dominance of key grazers may have a strong potential to alter resource stoichiometry of available macronutrients such as nitrogen (N) and phosphorus (P) with subsequent consequences for the structure and elemental composition of the phytoplankton community. However, our knowledge on stoichiometric interactions in the planktonic food web of the West Antarctic Peninsula is limited.

The aim of this study is to elucidate the potential relation between changes in macrozooplankton (krill vs. salps) densities and the nutrient dynamics at the WAP. We therefore analyzed the Palmer LTER data set by testing for the correlation between krill and salp density and the dissolved N and P pool, using a linear mixed effects model approach. The outcome of this model indicated that an increasing salp:krill ratio is significantly associated with higher dissolved N: P. In order to assess the potential mechanisms behind this pattern, we further conducted a field survey aboard the RV Polarstern in austral fall 2018, as well as laboratory experiments manipulating the presence of krill and salps in a plankton community. Preliminary results from the experiments revealed a significant impact of krill and salps on N and P recycling, resulting in higher dissolved N:P ratios in the presence of salps.

Our study demonstrates the importance of a comprehensive assessment of plankton community structure and biogeochemistry in order to predict how future changes in the composition of key species might modify stoichiometric dynamics in the planktonic food web of the WAP.