Trajectories of Changes in Phytoplankton Biomass, Phaeocystis globosa and Diatom (incl. Pseudo-nitzschia sp.) Abundances Related to Nutrient Pressures in the Eastern English Channel, Southern North Sea

Abstract

The phytoplankton compartment is particularly reactive to changes in nutrient concentration and is used as a quality indicator. Using a simple numerical approach, the response of emblematic harmful taxa from the eastern English Channel and southern North Sea to changes in nutrient inputs was studied. The method is based on a diachronic approach using averaged maxima over sliding periods of six years (1994–2018). This gave a final dataset containing pairs of points (number of years) for explained and explanatory variables. The temporal trajectory of the relationship between each pair of variables was then highlighted. Changes were represented as long-term trajectories that allowed a comparison to a reference/average situation. In addition, the relevance of the use of Phaeocystis globosa and the Pseudo-nitzchia complex as eutrophication species indicators was tested. Results showed a significant shift in the 2000s and different trajectories between diatoms and P. globosa abundances in response to changes in Dissolved Inorganic Nitrogen (DIN). The contrasting ecosystems under study reacted differently depending on the initial pressure. While a return to good ecological status does not seem feasible in the short term, it seems that these ecosystems were in an unstable intermediate state requiring continued efforts to reduce nutrient inputs.

Bi-decadal variability in physico-biogeochemical characteristics of temperate coastal ecosystems: from large-scale to local drivers

“…Despite this overall trend in the studied ecosystems and similar abiotic drivers of changes (Figs.2 and 3), different responses of individual ecosystems were observed in terms of magnitude, timing and even direction of the trajectories. Local discrepancies from an overall pattern are common, especially in coastal ecosystems that can face strong local drivers as seen in relationships between dissolved inorganic nitrogen and chlorophyll-a(Lefebvre & Dezécache 2020). Furthermore, no overall common pattern may be found highlighting the role of local over global drivers in controlling the functioning of coastal ecosystems(Talarmin et al 2016, Carstensen & Duarte 2019.…”

First Comprehensive Quantitative Multi-Parameter Assessment of the Eutrophication Status from Coastal to Marine French Waters in the English Channel, the Celtic Sea, the Bay of Biscay, and the Mediterranean Sea

“…The link between nutrient enrichment and the incidence of Harmful Algal Blooms (HAB, toxic, or high biomass) is still under investigation. Indeed, depending on the location and environmental conditions, some studies indicate that occurrence of HAB is linked to light, temperature, salinity, or climate and weather processes, rather than nutrient flux [41], whereas others allow us to consider with more confidence that the abundance of some HAB, like Phaeocystis globosa, is related to the concentration of DIN [42]. As a consequence, D5C3 criterion was not used in the French eutrophication assessment.…”

Optical Detection of Harmful Algal Blooms in the Belgian Coastal Zone: A Cautionary Tale of Chlorophyll c3

“…The haptophyte Phaeocystis globosa (Prymnesiophyceae) forms dense annual spring blooms in the Belgian Coastal Zone (BCZ) and adjacent regions of the southern North Sea (Cadée and Hegeman, 2002;Breton et al, 2006;Lefebvre and Dezécache, 2020). It has a complex life cycle transitioning between free-living haploid or diploid flagellated cells and colonial cells embedded in a mucilaginous matrix (Rousseau et al, 2013).…”

“…Weather and climate have a considerable influence on interannual variation of bloom intensity (e.g., Gieskes et al, 2007). However, long term analysis (Lefebvre and Dezécache, 2020) and ecosystem simulations (Lancelot et al, 2014) support the association between eutrophication and the intensification of P. globosa blooms beyond the threshold for healthy ecosystem function (Lancelot et al, 2009). The connection of bloom biomass with exported nutrients from land is also indicated by the observed and modeled relation of salinity, as a tracer for riverine nutrients, with the spring peak chlorophyll a concentration (Desmit et al, 2015).…”

“…The connection of bloom biomass with exported nutrients from land is also indicated by the observed and modeled relation of salinity, as a tracer for riverine nutrients, with the spring peak chlorophyll a concentration (Desmit et al, 2015). The monitoring of P. globosa blooms is therefore used for managing its potential impact and for evaluating the system's response to changes in environmental policies (Lancelot et al, 2011;Lefebvre and Dezécache, 2020) and climate (Gieskes et al, 2007).…”