Long-term changes of the trophic status in transitional ecosystems of the northern Adriatic Sea, key parameters and future expectations: The lagoon of Venice as a study case

Abstract

The determination of the trophic status of transitional ecosystems from the physico-chemical and biological point of view is one of the requirements of the European Water Framework Directive (WFD 2000/60/EC). In Italy, its determination is implemented by the Regional Agencies for Environmental Protection (ARPAs) that have activated multi-annual monitoring programs. However, as the availability of funds is increasingly scarce, the number of environmental parameters to detect environmental changes should be conveniently managed. The high number of environmental parameters, nutrient and macrophyte datasets available for the LTER-Italia site “Venice lagoon” can be an useful tool to analyze the trophic changes over recent years and to foresee environmental evolutions. Nutrient data on a spatial basis have been available since 1948, whereas macroalgal maps date back to 1980. The aim of this paper is to highlight the changes of the trophic status of the lagoon since the middle of the 20th century by considering the concentrations of nutrients in the surface sediments and in the water column, the variation of some physico-chemical parameters and the biomass of macroalgae and also to foresee the way it will possibly evolve. In fact, after many anthropogenic impacts that in the second half of the 20th century affected the lagoon, starting from the year 2010, the ecological status is progressively improving. Nutrients show a significant reduction both in the water column and in surface sediments, and the macrophytes are represented by species of higher ecological value while the opportunistic species such as the Ulvaceae are in strong regression.

Long-term dynamics in nutrients, chlorophyll <em>a</em> and water quality parameters in the Lagoon of Venice

“…Increasing water transparency in the period 1998-2007 has been attributed to declining river discharge into the LoV (Bernardi Aubry et al 2013). More recently (2011-2014), the reduction in clam farming and the resurgence of macroalgae and seagrasses have also been cited as causes (Sfriso et al 2019). The overall increase in transparency during the period considered in this study is shown in Tables 2 and 3. Mean relative oxygen increased significantly and since 2012 has never fallen below 110%, although these levels are still way below those observed in the late 1980s and early 1990s.…”

“…The overall increase in transparency during the period considered in this study is shown in Tables 2 and 3. Mean relative oxygen increased significantly and since 2012 has never fallen below 110%, although these levels are still way below those observed in the late 1980s and early 1990s. Indeed, that period saw average saturation levels of up to 250% in the central (Sfriso et al 2019) and northern lagoons (Bianchi et al 2000) due to the massive development of macro-and microalgae.…”

“…The last few decades have seen substantial changes in the LoV's morphological and hydrodynamic characteristics, together with significant variations in the amounts of pollutants and nutrients entering its waters (Solidoro et al 2010, Sfriso et al 2019). In the 1970s and 80s, when nutrient loads were at their highest, there was huge growth of Ulvaceae, and phytoplankton blooms were frequent, especially in the central basin, accompanied by a decline in seagrasses.…”

Settlement and Spreading of the Introduced Seaweed Caulacanthus okamurae (Rhodophyta) in the Mediterranean Sea

“…In the 1980s, the gross production of nuisance macroalgae, mostly Ulvaceae, was about 18.5 million tons on a fresh weight (fw) basis [24]. In the following years, nutrient concentrations and macroalgal production strongly declined and, currently, they are one order of magnitude lower [25]. Water temperature ranges between −0.5 • C (with glaciation of canals and the inner areas between salt marshes) and over 40.0 • C (July 2019) in very choked areas.…”

Editorial

“…The long-term analysis of various ecosystem components has also proved to be useful to identify key variables that may allow environmental dynamics to be followed in view of possible future limitations of sampling or analytical efforts. This is the case of tintinnids for marine microzooplankton (Monti-Birkenmeier et al 2019), taxonomic surrogates for benthic assemblages in transitional ecosystems (Pitacco et al 2019) and nutrients, such as nitrites and reactive phosphorus for the trophic status of the Venice Lagoon (Sfriso et al 2019). The value of LTER investigations, generally conducted at a single site, is further enhanced by expanding the spatial scale of the observations in a comparative effort, which is especially useful for the prediction of future changes in marine systems in response to climate variability and ocean warming (Kröncke et al 2019).…”