A model-based projection of historical state of a coastal ecosystem: Relevance of phytoplankton stoichiometry

Abstract

We employed a coupled physical-biogeochemical modelling framework for the reconstruction of the historic (H), pre-industrial state of a coastal system, the German Bight (southeastern North Sea), and we investigated its differences with the recent, control (C) state of the system. According to our findings: i) average winter concentrations of dissolved inorganic nitrogen and phosphorus (DIN and DIP) concentrations at the surface are ∼70-90% and ∼50-70% lower in the H state than in the C state within the nearshore waters, and differences gradually diminish towards off-shore waters; ii) differences in average growing season chlorophyll a (Chl) concentrations at the surface between the two states are mostly less than 50%; iii) in the off-shore areas, Chl concentrations in the deeper layers are affected less than in the surface layers; iv) reductions in phytoplankton carbon (C) biomass under the H state are weaker than those in Chl, due to the generally lower Chl:C ratios; v) in some areas the differences in growth rates between the two states are negligible, due to the compensation by lower light limitation under the H state, which in turn explains the lower Chl:C ratios; vi) zooplankton biomass, and hence the grazing pressure on phytoplankton is lower under the H state. This trophic decoupling is caused by the low nutritional quality (i.e., low N:C and P:C) of phytoplankton. These results call for increased attention to the relevance of the acclimation capacity and stoichiometric flexibility of phytoplankton for the prediction of their response to environmental change.

Assessing the Effects of WFD Nutrient Reductions Within an OSPAR Frame Using Trans-boundary Nutrient Modeling

“…This suggests that light or P limitation surpass N limitation even under TN and winter DIN concentrations being about 25 % lower in that region in the WFD scenario. This is in agreement with Kerimoglu et al (2018), who used a high-resolution model of the southern North Sea and also found light limitation to play a major role in this region. Other studies also identified P as the main limiting nutrient in the inner German Bight (Lenhart et al, 2010;Emeis et al, 2015;Wakelin et al, 2015), which suggests that additional P reductions might be required to achieve the "good environmental status" in the German Bight.…”

“…In order to calculate the individual reductions in DIN and PON, we calculate the average DIN:PON ratios for the individual German rivers during the period 2006-2012, as agreed on with stakeholders from the German Federal Environmental Agency, using the above described daily river dataset. With that, we translate the TN target concentration into target concentrations for DIN and PON and calculate the reduction levels based on their 2006-2012 average concentrations, following Kerimoglu et al (2018). As DIN:PON ratios vary between the different German rivers and throughout the seasonal cycle, we obtain different reductions levels for these rivers as well as for DIN and PON.…”

“…An interesting feature occurs in the inner German Bight, where a region of only small changes in Chl-a extends northwestward from the Elbe River inlet, although changes in TN and winter DIN are significant. This could relate to the high turbidity in the Elbe River plume, causing light limitation to be the main controlling factor of primary productivity (Kerimoglu et al, 2018). Additionally, the negligence of riverine P load reductions in the WFD scenario could play a role here, as P limitation likely limits spring primary production in the coastal North Sea (Billen et al, 2011;Emeis et al, 2015).…”

“…Pre-industrial conditions are often considered pristine due to the comparably small anthropogenic impact. They are usually defined as the status of the mid-or late 19th century (e.g., Serna et al, 2010;Kerimoglu et al, 2018), as reliable external nutrient inputs can only be dated back until then (Schöpp et al, 2003;Hirt et al, 2014). However, Desmit et al (2018) derived "truly pristine" conditions before any anthropogenic disturbance using a catchment model for western Europe.…”

Interactive impacts of meteorological and hydrological conditions on the physical and biogeochemical structure of a coastal system

“…As in Kerimoglu et al (2017b), assimilated and un-assimilated fractions of the ingested prey by each zooplankton j are determined by the assimilation efficiency X j (B23),B24), which is continuously adjusted (as in Grover, 2002), such that the zooplankton can maintain their homeostatic elemental composition. Here this scheme was extended to multiple nutrients, i.e., N and P, and X are calculated iteratively, similar to that in (Kerimoglu et al, 2018). Starting from each X set to default values 470 (Table B5), if P to be ingested would be less than the amount required to match the ingested C, C is down-regulated, and vice versa:…”