Phytoplankton–Macrophyte Interaction in the Lagoon of Venice (Northern Adriatic Sea, Italy)

Abstract

The coexistence of phytoplankton and macrophytes in the Lagoon of Venice (Northern Adriatic Sea, Italy) was investigated using in situ data collected monthly as part of International Long Term Ecosystem Research (LTER), together with satellite imagery for the period 1998–2017. The concentrations of chlorophyll a and hydrochemical parameters were measured in three areas of the lagoon, where the expansion of well-developed stands of submerged vegetation was observed by remote sensing. Our results suggest interaction between phytoplankton and macrophytes (macroalgae and seagrasses) in the last few years of the time series, evidenced by decreasing chlorophyll a concentrations in the vicinity of the macrophyte stands. The integration of LTER and remotely sensed data made it possible to evaluate the interaction of macrophytes and phytoplankton at the ecosystem scale for the first time in the Lagoon of Venice.

Phytoplankton Dynamics and Water Quality in the Venice Lagoon

“…This paper could be considered as a "third episode of a series" addressing the changes of the water quality in the VL within the same 20-year period (1998-2017), from different points of view. Actually, 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.…”

“…These changes seem to be related to several factors, such as the application of more stringent environmental regulations concerning farm wastes in the watershed, declining river discharge, and a reduction in clam harvesting in the lagoon. At the same time, an upswing of macrophytes has been observed [14,18], evidencing the complex interrelations between the different primary producers, which can exploit the LV habitat heterogeneity, and the different trophic and water quality conditions.…”

“…In the last two decades, a general improvement of the water quality of the VL has been documented [13,14,17]. In particular, water transparency and oxygen have increased, while nutrients (ammonia and nitrate) and chlorophyll-a have decreased.…”

“…The VL is the largest Italian lagoon and is one of the research sites belonging to the Italian Long-Term Ecological research network LTER-Italy (www.lteritalia.it (accessed on 4 October 2021)). Data on the phytoplankton communities in the lagoon have been available sporadically since the 1970s, but they have only been gathered regularly and with comparable methods of investigation since the late 1990s [12][13][14]. The water environment of the VL is turbulent, nutrient-enriched, and open to other connected systems, i.e., the land, the sea and the sediments: these broad ecological features are those mainly affecting the prevalent phytoplankton community composition, its seasonal cycles and its morphological traits [13,15].…”

Multiple Evidence for Climate Patterns Influencing Ecosystem Productivity across Spatial Gradients in the Venice Lagoon

“…According to backand forecasting scenarios by [42], the lagoon shifted from a scenario where production exceeded respiration (beginning of 1900) to one where respiration was the predominant player (current and future), and this could be attributed to the presence and activity of Manilla clam farming. The role of other species such as macrophytes, likely to become more abundant with rising temperatures (e.g., Ulvaceae [43]), should also be considered with ad-hoc abundance surveys for the whole lagoon, which may become more accessible with the use of new technologies (e.g., [44]).…”