64PE366 - CHARLET Introduction

Primary production by phytoplankton (unicellular algae; see photos below) provides the basis of marine food webs and is strongly determined by nutrient and light availability.

Measures against eutrophication (enrichment with mainly nitrate and phosphate salts) have mainly led to a reduction in phosphorus inputs into coastal seas like the North Sea, whereas nitrogen and silica loadings were much less reduced. This has resulted in major changes in the relative availability of different nutrients, and there is currently substantial disagreement whether phytoplankton growth in coastal waters is limited by nitrogen, phosphorus, or light. Furthermore, resource-mediated changes in the cellular composition of phytoplankton will have major implications for their nutritional quality for zooplankton (predators), with effects that may cascade throughout the entire aquatic food web (see image below).


In this project, we determine the limiting factors for phytoplankton growth in the North Sea, and examine how these limiting factors affect the food quality and species composition of the phytoplankton. We use novel approaches to assess resource limitation, and implement our results in phytoplankton competition mathematical models. Furthermore, we investigate how the transfer of primary production to the classical zooplankton-based food chain versus the viral loop (viral infection results in cell death and subsequent breakdown of the cellular debris, thereby recycling inorganic nutrients back into the water column) is affected by shifts in the phytoplankton community and their food quality.

The project combines mathematical models, laboratory studies and field work during cruises with the R/V Pelagia in two contrasting areas of the North Sea: the productive coastal area with relatively high nutrient inputs from rivers and the central North Sea with much reduced nutrient levels especially during summer.

The project involves 4 cruises in the North Sea during spring and summer, of which the current one is the third cruise. The proposed work will offer key insights into the impact of changes in resource limitation on the carrying capacity of the North Sea.