Abstract
We analyzed the phytoplankton abundance and community structure monthly over a 20-year period (1998–2017) at five stations in the Venice lagoon (VL), one of the sites belonging to the Long-Term Ecological Research network of Italy (LTER-Italy). We focused on phytoplankton seasonal patterns, inter-annual variability and long-term trends in relation to water quality. Diatoms numerically dominated (ca. 60% on average), followed by nanoflagellates (37%), while coccolithophorids and dinoflagellates contributed less than 2%. We observed distinct seasonal and inter-annual changes in the abundance and floristic composition of the phytoplankton groups, whilst no clear long-term trend was statistically significant. We also assessed the water quality changes, applying to our dataset the multimetric phytoplankton index (MPI), recently officially adopted by Italy to accomplish the water framework directive (WFD) requirements. The index evidenced a temporal improvement of the water quality from “moderate” to “good” and allowed us to confirm its reliability to address the changes in the water quality, not only spatially—as previously known—but also for following the yearly time trends. Overall, our results highlight the importance of long-term observations, for understanding the variability in the phytoplankton communities of the lagoon as well as the relevance of their use to test and apply synthetic descriptors of water quality, in compliance with the environmental directives.
Highlights
Phytoplankton is the only planktonic element included as a water quality indicator in the water framework directive (WFD), which enforced an integrated and coordinated framework for the monitoring of ecological status and the management of transitional waters, based on key biological elements [3]
This paper could be considered as a “third episode of a series” addressing the changes of the water quality in the Venice Lagoon (VL) within the same 20-year period (1998–2017), from different points of view
Two previous papers analyzed the VL water quality considering, respectively, (i) the abiotic parameters and the chlorophyll-a [17], and (ii) the relations between phytoplankton and macrophytes [14]. They both evidenced an overall improvement of the lagoon water quality, which seemed to enter a new phase in the most recent years
Summary
An intrinsically high seasonal and interannual variability characterize the phytoplankton communities in coastal transitional environments [1,2], such as the Venice Lagoon (VL). Some phytoplankton-related variables, such as taxonomic composition, abundance, biomass, frequency and intensity of blooms, and cell size spectra have been proved relevant for the definition and classification of the water quality of coastal transitional environments, in order to implement the directive [4,5,6,7]. A reliable tool for the classification of transitional waters, the multimetric phytoplankton index (MPI), has been formulated by Facca and colleagues [8]: the index, based on phytoplankton biodiversity, chlorophyll-a and bloom frequency, was developed in the VL and successfully applied to compare specific areas in the lagoon. The MPI has been successfully applied in some Mediterranean lagoons in Sardinia [9], and it is currently used to assess the status of the Italian, Greek and Croatian transitional waters [10,11]
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