Abstract
One of the most striking patterns at the land–ocean interface is the massive increase of chlorophyll-a (CHL) from continental shelves towards the coast, a phenomenon that is classically linked to physical features. Here I propose that the coastal–offshore CHL gradient in a shallow sea has biological origins related to phytoplankton mortality that are neglected in state-of-the-art biogeochemical models. I integrate a trait-based ecosystem model into a modular coupling framework that is applied to the southern North Sea (SNS). The coupled model very well reproduces daily, seasonal and inter-annual (2000-2014) dynamics and meso-scale patterns in macronutrients, zooplankton biomass, and CHL as observed in situ and by remote sensors. Numerical experiments reveal that coast–offshore CHL gradients may predominantly arise from a trophic effect as resolved by an increase in carnivorous grazing towards shallow waters. This carnivory gradient reflects higher near-coast abundance of juvenile fish and benthic filter feeders. Furthermore, the temporal evolution of CHL can be much affected by viral infection as a fast-responding loss process at intermediate to high phytoplankton concentrations. Viral control in the model also prevents excessive and unrealistic blooms during late spring. Herbivores as often only ecological factor considered for explaining the spatio-temporal phytoplankton distribution are in this study supplemented by pathogens as well as pelagic and benthic carnivores as powerful agents, which are barely represented in current modeling but can mediate physical drivers of coastal ecosystems.
Highlights
Marine primary production by unicellular autotrophs makes the fundamental basis of both biogeochemical cycling and oceanic to coastal food-webs [1, 2, 3]
Nutrient concentrations calculated by Model for Adaptive Ecosystems in Coastal Seas (MAECS) in average exhibit a steep cross-shore gradient, which diminishes during summer, especially for dissolved inorganic phosphorus (DIP)
DIP varies cross-shore by a factor of 2-8, which is much less compared to the gradient in Dissolved inorganic nitrogen (DIN) that covers two orders of magnitude over a short distance
Summary
Marine primary production by unicellular autotrophs makes the fundamental basis of both biogeochemical cycling and oceanic to coastal food-webs [1, 2, 3]. The complete model setup is available from a separate repository sf.net/p/mossco/setups, where all of the configuration and data pertaining to this publication are freely available, with the exception of the meteorological forcing fields. These are available at no cost on request from the coastDat data base for the assessment of long-term changes by Helmholtz-Zentrum Geesthacht at www.coastdat.
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