Spatio-temporal dynamics of North Sea plankton during low productivity seasons and implications for higher trophic levels

Abstract

Plankton forms the foundation of the food webs and, therefore, to manage the ecosystem and fish stocks effectively, it is essential to understand the interactions between the different compartments of the plankton community and the associated environmental drivers. In the North Sea, winter is considered a low productivity season and it is a crucial time for the survival and growth of marine organisms, including early life stages of many fish species, such as the Atlantic herring (Clupea harengus). Nevertheless, despite the recognized importance of this season, our understanding of plankton dynamics is still very limited underlining the need for further investigations on the plankton standing stocks and community structure, which will constitute the prey fields for larger trophic levels. In this thesis, I have evaluated the spatio-temporal dynamics of the North Sea plankton community during autumn and winter of different size fractions, the Protozoo- (PZP), the Microzoo- (MZP) and Mesozooplankton (MesoZP) community on a broad spatial scale across several years. The analysis of the PZP community revealed a homogenous community across the central North Sea with a general north-south gradient in terms of abundance of the the two main groups, dinoflagellates and ciliates. We identified only marginal effects of environmental drivers that may affect PZP shifts in the taxonomic and size composition of the community. To accelerate the time-consuming plankton identification process, we developed an automated Dynamic Optimization Cycle (DOC) pipeline to frequently improve and adapt the accuracy of a plankton image classification model, which helped us to analyze the MZP and MesoZP community in the two main spawning grounds of herring in autumn and winter. By combining two different nets, we provided more accurate estimates of the MesoZP community and a novel dataset of MZP abundances and biomasses for autumn and winter. The application of two different analytical approaches, a taxonomic and a size-based approach identified distinct environmental drivers related to water masses affecting the community, such as salinity and temperature in the respective spawning ground. The generated slopes from the broad groups included in the size-based approach where then used to explore the feeding conditions of the herring larvae in the respective spawning ground and season via physiological modeling. The growth of small larvae in both areas was limited by food availability, even under favorable feeding conditions. In Downs, even larger larvae (>13 mm) were predicted to experience food limitation during winter, while in Buchan/Banks, they were able to approached their maximal growth capacity in autumn. However, in both areas the larvae showed generally better growth performance and survival when their prey fields were extended beyond the “traditional” prey items such as copepods and nauplii, including microplankters in their diet. These findings highlight the importance of understanding the dynamics of the PZP, MZP and MesoZP communities, stressing the need for further monitoring within larval fish surveys in wintertime, especially given the ongoing low recruitment of North Sea herring. The findings of this study contribute to a better understanding of the standing stock of plankton and its relationship with environmental drivers in temperate shelf seas. Therefore, this thesis offers additional knowledge for management of the North Sea ecosystem and fish stocks, and underscores the need for further investigation into the dynamics of plankton communities during low productivity periods.