Changes in chlorophyll concentration and phenology in the North Sea in relation to de‐eutrophication and sea surface warming

Abstract

At least two major drivers of phytoplankton production have changed in recent decades in the North Sea: sea surface temperature (SST) has increased by~1.6 C between 1988 and 2014, and the nitrogen and phosphorus loads from surrounding rivers have decreased from the mid-1980s onward, following reduction policies. Long time series spanning four decades of nutrients, chlorophyll (Chl), and pH measurements in the Southern and Central North Sea were analyzed to assess the impact of both the warming and the deeutrophication trends on Chl. The de-eutrophication process, detectable in the reduction of nutrient river loads to the sea, caused a decrease of nutrient concentrations in coastal waters under riverine influence. A decline in annual mean Chl was observed at 11 out of 18 sampling sites (coastal and offshore) in the period 1988-2016. Also, a shift in Chl phenology was observed around 2000, with spring bloom formation occurring earlier in the year. A long time series of pH in the Southern North Sea showed an increase until the mid-1980s followed by a rapid decrease, suggesting changes in phytoplankton production that would support the observed changes in Chl. Linear correlations, however, did not reveal significant relationships between Chl variability and winter nutrients or SST at the sampling sites. We propose that the observed changes in Chl (annual or seasonal) around 2000 are a response of phytoplankton dynamics to multiple stressors, directly or indirectly influenced by deeutrophication and climate warming.

Less Nutrients but More Phytoplankton: Long-Term Ecosystem Dynamics of the Southern North Sea

“…As a consequence, a decline in Chl or primary production in the SNS should be expected due to the reduced riverine loads of nutrients after the late 1980s. This common view is supported by studies that report decreasing Chl at coastal sites (van Beusekom et al, 2009;Desmit et al, 2019). These studies referred to monitoring stations located at Sylt ("L" in Supplementary Figure S3) and very near-shore along the northern (T-10 in Supplementary Figure S3) and southern (NW-02, GR-06 in Supplementary Figure S3) Dutch coast.…”

“…Long-term in situ observations of phytoplankton and nutrient concentration are available for a number of sites (Cadee and Hegeman, 2002;Smaal et al, 2013;Desmit et al, 2019), but prior model studies of long-term biomass dynamics (Daewel and Schrum, 2013;Lynam et al, 2017;Capuzzo et al, 2018) have been validated only against sparse data sets, which lack regional details and long-term variability. Moreover, state-of-the-art coupled biogeochemical models often come with a relatively coarse spatial resolution and rarely resolve strong gradients in phytoplankton community composition and ecophysiology.…”

Changing circumpolar distributions and isoscapes of Antarctic krill: Indo‐Pacific habitat refuges counter long‐term degradation of the Atlantic sector

“…We compared sector-based mean krill densities (as described above for the CHINARE cruise), both for the April-October period and the December-February (midsummer) periods of each era. In our second approach to looking at time trends, we used a series of Mann-Kendall tests (Desmit et al 2020) to assess the direction and significance of time trends in krill density in each sector. These nonparametric tests are suited to data such as KRILLBASE, because they are not sensitive to the treatment of gaps in time series, nor are they sensitive to the exact type of data transformation; valuable for highly skewed distributions such as those of krill.…”

A century of coping with environmental and ecological changes via compensatory biomineralization in mussels

“…Our models indicated formation of more variable surface regimes along the Belgian coast over the last century, with temporal patterns depending on the descriptor considered (Figure 3a; Figure S6; Table 1; Table S5). Long‐term increases of SST with warmer spring and summer temperatures are in line with global ocean trends (Desmit et al., 2020; Kirtman et al., 2013). No change in long‐term and seasonal SSS patterns was detected, reflecting expected long‐term stability in local water cycles for areas with intermediate salinity (mid‐latitudes; Kirtman et al., 2013).…”