Phytoplankton Dynamics and Water Quality in the Venice Lagoon

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.

Differences and similarities in the phytoplankton communities of two coupled transitional and marine ecosystems (the Lagoon of Venice and the Gulf of Venice - Northern Adriatic Sea)

“…Both belong to LTER-Italy (www.lteritalia.it; Bastianini et al, 2021;Camatti et al, 2021;Capotondi et al, 2021), the national Long-Term Ecological Research (LTER) network. We therefore have consistent data and knowledge, regarding phytoplankton assemblages and related abiotic factors in particular (Bernardi Aubry et al, 2012;Bernardi Aubry et al, 2013;Acri et al, 2020;Bernardi Aubry et al, 2020;Bernardi Aubry et al, 2021), which may provide useful background information for answering specific questions about the mechanisms that shape community structure. The LoV is separated from the GoV by two long barrier islands, water exchange with the sea being enabled by three wide inlets.…”

“…We performed various types of analysis in order to: (i) identify the prevalent features and seasonal patterns of abiotic factors and chl a, (ii) assess and compare taxonomic composition and diversity at each station and (iii) identify the generalist and specialist taxa. We adopted a conventional division of the seasons (Bernardi Aubry et al, 2013;Bernardi Aubry et al, 2021) as follows: January-March = winter; April-June = spring; July-September = summer; October-December = autumn.…”

“…In both areas, phytoplankton saw the highest chl a and abundance in spring and summer. However, the seasonal pattern of phytoplankton abundance in the LoV is prevalently unimodal, with a summer peak, which can be considered typical of this environment (Bernardi Aubry et al, 2013;Bernardi Aubry et al, 2021) and of temperate shallow and nutrient-enriched enclosed coastal ecosystems in general (Cebrian and Valiela, 1999;Cloern and Jassby, 2008;Winder and Cloern, 2010;Zingone et al, 2010b). Indeed, this feature could be related to the high nutrient concentrations in the LoV, which reduces the seasonal constraint on phytoplankton growth, thus making the seasonal climate-based cycles of temperature and light the main driver of phytoplankton abundance.…”

“…Most of the diatom taxa found in the LoV belong to the benthic habitat, having been resuspended in the water column from sediments. Close benthic-pelagic coupling is a well-known and typical feature of the LoV (Facca et al, 2002;Facca and Sfriso, 2007;Bernardi Aubry et al, 2013;Bernardi Aubry et al, 2017;Bernardi Aubry et al, 2021) and other shallow transitional ecosystems (Brito et al, 2012;Rubino and Belmonte, 2021), where the resuspension of microphytobenthos represents an essential element of phytoplankton dynamics. The constant interactions between water column and sediment blur the boundary between the two habitats, so that the microalgal populations recorded in the water column do not differ significantly from those found in the upper sediment layer (Facca et al, 2002).…”

Diversity of Free-Living and Particle-Attached Prokaryotes in a River-Influenced Coastal Area of the Northern Adriatic Sea

“…The haline stratification segregates phytoplankton at surface, triggering and controlling phytoplankton blooms, especially in the frontal area of the river delta (Mangoni et al, 2008). In general, it has been observed that total phytoplankton abundances decreased over the past two decades with a significant shift in 2003 (Cozzi and Giani, 2011;Djakovac et al, 2012;Bernardi Aubry et al, 2021), probably due to the reduction in the total load of freshwater discharge and related nutrient loads.…”