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

Alain Lefebvre,Camille Dezécache

doi:10.3390/jmse8060401

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.

Highlights

The importance of phytoplankton at the level of aquatic ecosystems is no longer in doubt
Coastal areas concentrate human activities benefiting from incomes deriving from sea uses, as well as areas of high phytoplankton concentrations leading to environmental disturbances or eventual toxicity of sea products
This gives inter-site trajectories with very different directions from the general relationship previously established for the whole period of interest (Figure 12): Besides a decrease in Dissolved Inorganic Nitrogen (DIN) concentrations, diatom concentrations increase for all sites during the period considered

Summary

Introduction

The importance of phytoplankton at the level of aquatic ecosystems is no longer in doubt (base of the food web, key component of the structure and function of coastal marine ecosystems and carbon and nutrient budgets, etc.). While it is always desirable to obtain the most precise information about species composition and hydrological parameters with the highest possible sampling frequency, these datasets still lack sufficient historical coverage to assess the environmental consequences of climate change or human activities, such as eutrophication, contrary to classic low-frequency in-situ monitoring. In addition to their long historical coverage, these low-frequency sampling techniques are often applied over contrasted ecosystems. Our objective is to confirm or deny sensitivity of Phaeocystis to the management of nutrient inputs, or linkage in space and time to the availability of anthropogenic nutrients

Materials and Methods

Relationship between DIN and Diatom Concentrations

Changes in the Competitive Advantage of Diatoms Versus Phaeocystis

Discussion

Change Towards an Unstable Status

Limitations of Our Approach—Advice for Monitoring and Indicator Development