Abstract

Phosphorus (P) is a major driver of eutrophication, especially in anthropogenically impacted coastal waters, and determining its bioavailability is important for providing a good estimation of the eutrophication potential in aquatic systems. Therefore, we observed the bioavailability of P in four laboratory experiments on water samples collected in March, June, September, and December 2018. In the experiments, all P fractions of the sampled water were investigated in three treatments (“unfiltered” and “10 μm”- and “1.2 μm”-filtered). The bioavailability (utilization by organisms within several days) ranged from 9 to 100% for dissolved P, and 34 to 100% for particulate P. However, one of the particulate P fractions was bound in biomass and therefore was not directly bioavailable. The conditions in the March experiment represented a natural spring bloom with a residual potential for planktonic growth. In June and September, the nutrients needed for growth were depleted in the different treatments. In December, a spring bloom was simulated by the laboratory conditions. Preferential P uptake by a specific group of organisms could not be observed directly, although a trend of higher utilization of dissolved P by heterotrophic bacteria was observed. In conclusion, the bioavailable P (sum of dissolved P fractions and one particulate P fraction) accounted for between 20 and 94% of the total P. Consequently, our experiments demonstrated that the commonly monitored P fractions lead to an underestimation of the bioavailable P and thus of potential for eutrophication in aquatic systems, too.

Highlights

  • Connection Between P and EutrophicationPhosphorous (P) is essential for life because it is a component of important biomolecules (e.g., DNA and ATP)

  • In the “unfiltered” treatment, the maximum total P (TP) concentration was measured in June, and the minimum was measured in September

  • The biological community in this study contained a large proportion of chlorophyll a (Chla), which is an indicator of the presence of primary producers if the amount of Chla and the total particulate C content are correlated

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Summary

Introduction

Connection Between P and EutrophicationPhosphorous (P) is essential for life because it is a component of important biomolecules (e.g., DNA and ATP). Eutrophication is a global problem in limnic and marine systems (e.g., Yang et al, 2008; Chrislock et al, 2013; Jessen et al, 2015). Since the 20th century, anthropogenic activity has led to excessive input of both P and N into aquatic systems and caused shifts in the trophic status of European waters, such as the Baltic Sea, which shifted from an oligo- to a eutrophic system (Gustafsson et al, 2012). To address the eutrophication problem, regulations such as the Water Framework Directive (WFD, 2000/60/EC), the Marine Strategy Framework Directive (MSFD, 2008/56/EC), and the Helsinki Commission’s (HELCOM) Baltic Sea Action Plan (BSAP, HELCOM, 2007, 2013) were adopted to achieve higher water quality. The level of eutrophication remains high in 95% of the Baltic Sea (LUNG, 2016; HELCOM, 2018a,b), and further reductions of the nutrient inputs are needed. The catchment areas of the rivers that discharge into the Baltic Sea have become a focus of HELCOM’s efforts

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