Abstract
Satellite-derived estimates of ocean color variables are available for several decades now and allow performing studies of the long-term changes occurred in an ecosystem. A daily, gap-free analysis of chlorophyll (CHL) and suspended particulate matter (SPM, indicative of light availability in the subsurface) at 1 km resolution over the Greater North Sea during the period 1998–2020 is presented. Interannual changes are described, with maximum average CHL values increasing during the period 1998–2008, a slightly decreasing trend in 2009–2017 and an stagnation in recent years. The typical spring bloom is observed to happen earlier each year, with about 1 month difference between 1998 and 2020. The duration of the bloom (time between onset and offset) appears also to be increasing with time, but the average CHL value during the spring bloom does not show a clear trend. The causes for earlier spring blooms are still unclear, although a rising water temperature can partially explain them through enhanced phytoplankton cell division rates or through increased water column stratification. SPM values during winter months (prior to the development of the spring bloom) do not exhibit a clear trend over the same period, although slightly higher SPM values are observed in recent years. The influence of sea surface temperature in the spring bloom timing appears to be dominant over the influence of SPM concentration, according to our results. The number of satellites available over the years for producing CHL and SPM in this work has an influence in the total amount of available data before interpolation. The amount of missing data has an influence in the total variability that is retained in the final dataset, and our results suggest that at least three satellites would be needed for a good representation of ocean color variability.
Highlights
The North Sea is a semi-enclosed shallow shelf sea in northwestern Europe, and it is one of the most productive seas in the world (Ducrotoy et al, 2000)
23-Year North Sea DINEOF Reconstruction number of optimal EOFs depending on factors like the available data, the cloud coverage and the structures that are observed in the initial data
The multi-year dataset have been used in the frame of the EUfunded JMP-EUNOSAT project (Joint Monitoring Programme of the Eutrophication of the North Sea with Satellite data), to assess the use of satellite data to monitor the eutrophication in the North Sea with the help of satellite data, and a thorough validation has been realized in that project (Van der Zande et al, 2019a)
Summary
The North Sea is a semi-enclosed shallow shelf sea in northwestern Europe, and it is one of the most productive seas in the world (Ducrotoy et al, 2000). Despite de-eutrophication policies implemented since the 1990s, such as the EU Marine Strategy Framework Directive (MSFD) which aims at reaching a Good Environmental Status (GES) in European waters, the North Sea still receives relatively high nutrient inputs (nitrogen and phosphorous, Van der Zande et al, 2019b). This results in intense phytoplankton blooms occurring every year between March and October, with the southern parts of the North Sea, shallower and more affected by industrial and agricultural activities, presenting more intense blooms (Lancelot et al, 2005; Rousseau et al, 2013; Desmit et al, 2015, 2020). Phytoplankton spatial and temporal dynamics can be influenced by several factors, including the availability of nutrients and light, water temperature, and grazing (Capuzzo et al, 2017; Xu et al, 2020)
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