Abstract
Abstract. As the primary energy and carbon source in aquatic food webs, phytoplankton generally display spatial heterogeneity due to complicated biotic and abiotic controls; however our understanding of the causes of this spatial heterogeneity is challenging, as it involves multiple regulatory mechanisms. We applied a combination of field observation, numerical modeling, and remote sensing to display and interpret the spatial gradient of phytoplankton biomass in a Dutch tidal bay (the Eastern Scheldt) on the east coast of the North Sea. The 19 years (1995–2013) of monitoring data reveal a seaward increasing trend in chlorophyll-a (chl a) concentrations during the spring bloom. Using a calibrated and validated three-dimensional hydrodynamic–biogeochemical model, two idealized model scenarios were run: switching off the suspension feeders and halving the open-boundary nutrient and phytoplankton loading. Results reveal that bivalve grazing exerts a dominant control on phytoplankton in the bay and that the tidal import mainly influences algal biomass near the mouth. Satellite data captured a post-bloom snapshot that indicated the temporally variable phytoplankton distribution. Based on a literature review, we found five common spatial phytoplankton patterns in global estuarine–coastal ecosystems for comparison with the Eastern Scheldt case: seaward increasing, seaward decreasing, concave with a chlorophyll maximum, weak spatial gradients, and irregular patterns. We highlight the temporal variability of these spatial patterns and the importance of anthropogenic and environmental influences.
Highlights
As the most important energy source in aquatic systems, phytoplankton account for 1 % of the global biomass but create around 50 % of the global primary production (Boyce et al, 2010)
We applied a combination of field observation, numerical modeling, and remote sensing to display and interpret the spatial gradient of phytoplankton biomass in a Dutch tidal bay on the east coast of the North Sea
The 19-year chl-a time series illustrates the seasonal pattern of phytoplankton biomass in the Eastern Scheldt (Fig. 4)
Summary
As the most important energy source in aquatic systems, phytoplankton account for 1 % of the global biomass but create around 50 % of the global primary production (Boyce et al, 2010). Located at the land–ocean interface, estuarine– coastal systems, including estuaries, bays, lagoons, fjords, river deltas, and plumes, are relatively productive and abundant in phytoplankton (Carstensen et al, 2015). As the basis of the pelagic food web, phytoplankton have an immense impact on the biogeochemical cycles, water quality, and ecosystem services (Cloern et al, 2014). The standing stock of phytoplankton is a function of sources and sinks that are subject to both biotic and abiotic influences (Lancelot and Muylaert, 2011; Jiang et al, 2015). Phytoplankton growth is regulated by bottom-up factors such as nutrients, light, and temperature
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