Fifty years of ecological changes: Regime shifts and drivers in a coastal Mediterranean lagoon during oligotrophication

Abstract

Thau lagoon is a large Mediterranean coastal lagoons and it supports traditional shellfish farming activities. It has been subject to eutrophication leading to major anoxic events associated with massive mortalities of shellfish stocks. Since the 1970s, improvements have been made to wastewater treatment systems, which have gradually led to oligotrophication of the lagoon. The aim of our study was to determine how the decrease in nutrient inputs resulted in major ecological changes in Thau lagoon, by analysing five decades of time-series of observations on pelagic and benthic autotrophic communities. We were able to identify two periods during the oligotrophication process. Period 1 was considered a eutrophic period, characterised by the shift from seagrass dominance to dominance of red macroalgae. Period 2 (1993-2018), characterised by improved eutrophication status, was further divided into three: a transition phase (1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003) during which the water column continued to recover but the benthic community lagged behind in recovery and in partial resilience; a regime shift (2003)(2004)(2005)(2006), after which the water column became oligotrophic and seagrass began to recover (2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018). Considering anoxia crises as indicators of ecosystem resilience and resistance, we used a generalised linear model to analyse meteorological and environmental data with the aim of identifying the triggers of summer anoxia over the study period. Among the meteorological variables studied, air temperature had the strongest positive effect, followed by the period and wind intensity (both negative effects) and by rainfall in July (positive effect). The risk of triggering anoxia was lower in period 2, evidence for the increasing resistance of the ecosystem to climatic stress throughout the oligotrophication process. At the ecosystem scale and in the long term perspective, the ecological gains related to oligotrophication are especially important in the context of climate change, with more frequent and severe heat waves predicted.Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.

Internal Nutrient Loading Controls Macroalgal and Cyanobacterial Succession in a Coastal Lagoon Restored by Managed Realignment of Agricultural Land

“…The gradual change in macroalgal community structure from opportunistic green algal toward perennial brown and red algal dominance in Gyldensteen Coastal Lagoon during the 5-year study period was strongly controlled by the declining internal nutrient loading, particularly DIN availability. Similar nutrient-dependent recovery of perennial macroalgal communities has in recent years been observed in a variety of coastal environments (Pinedo et al, 2013;Rasmussen et al, 2015;Le Fur et al, 2019;Derolez et al, 2020). The moderate DON-based recovery of opportunistic green algae in early summer of 2018 (Figure 4) was apparently not strong enough to preclude substantial growth of the perennial brown (F. vesiculosus) and red algae (A. vermiculophylla).…”

Effects of experimental warming on small phytoplankton, bacteria and viruses in autumn in the Mediterranean coastal Thau Lagoon

“…In addition, these changes in the balance between growth and grazing rates over the course of the experiment were difficult to track due to the dilution experiment frequency applied. Second, the cytometric analyses of cyanobacteria also revealed Prochlorococcus-like cells, a genus that, to our knowledge, had never been reported in Thau Lagoon during the past 20 years (Bec et al 2005;Collos et al 2009;Trombetta et al 2019;Derolez et al 2020a). Nevertheless, the Prochlorococcus genus present in the Mediterranean Sea could potentially drift into the lagoon via the channels connecting it to the sea.…”

“…Such high abundances have only been reported three other times before in Thau Lagoon, as exceptional events in 2003exceptional events in , in 2006exceptional events in and, more recently, in 2018exceptional events in (Derolez et al 2020a). It appears that these high abundances of picocyanobacteria were concurrent with relatively warm years, suggesting that warming in Thau Lagoon naturally favours picocyanobacteria (Trombetta et al 2019;Derolez et al 2020a). It should be noted that at the very beginning of the experiment, the Synechococus-like abundance declined accordingly to the low growth and high grazing rates estimated on day 2.…”

The Collaborative Process in Environmental Projects, a Place-Based Coevolution Perspective

“…With regard to on-the-ground changes, they may be difficult to relate to specific research activities, but the results show the increase in the complexity of sociopolitical interactions through the evolution of the collaborative process. Many works today are tackling social-ecological outcomes, for example, concerning the Thau image [102], local observatories [110], or ecological impacts on the lagoon ecosystems [111,112]. The following discussion will be centered on the methodological contributions and mention these impacts.…”

“…The lagoon ecosystem is now more resistant to the threat of summer heat waves and anoxic crises [112], but the territoire is facing new issues driven by climatic change trends, which is expected to influence the observed recovery process. Some inspiring multidisciplinary approaches emerges for the study of the ecological restoration trajectory of coastal lagoons [115].…”