Effects of anthropogenic and natural disturbance on sedimentary organic matter quantity, nutritional quality and degradation in river, lagoon, and coastal marine ecosystems

Ecological disturbance can play a fundamental role in regulating habitats’ structure and biodiversity. Aquatic ecosystems, especially lagoons and marine coastal areas, appear to be more sensitive to anthropogenic and natural disturbances than terrestrial ones, whose ecological consequences are more tangible. Changes in the trophic status of aquatic ecosystems and biogeochemical cycling are recurrently being reported due to either natural or anthropogenic drivers, including climate change (CC). In this context, my doctoral thesis aimed at investigating the ecological effects of selected anthropogenic and natural disturbances on sedimentary organic matter (OM) quantity, biochemical composition, nutritional quality, and C degradation rates, with the aim of exploring the magnitude and direction of changes in the benthic trophic status of river, lagoon, and coastal marine ecosystems through experiments carried out in the field or in mesocosms. First, I investigated the possible effects on benthic trophic status of bottom trawling, one of the major anthropogenic disturbances to which marine benthic ecosystems are subjected worldwide. The study was conducted within an area located in the Bornholm Basin (Baltic Sea) in sites putatively representative of null, low, and medium trawling intensities down to 4 cm depth. The results confirm the capacity of bottom trawling to mix the sediment layers, ultimately causing an overall homogenization of the trophic status across the whole sediment column and altering benthic ecosystem functioning. Then, I focused on marine heatwaves (MHWs), since their intensification caused by CC is expected to increasingly affect coastal biogeochemistry. I investigated MHWs’ effects on sedimentary OM in a nearshore locality (NW Sardinia, Mediterranean Sea) receiving an artificial warm water plume generating temperature anomalies of 1.5-5.0 °C. Sediments were collected before and after 3 and 11 weeks from the initial plume release. I contend that MHWs’ intensification will affect not only species and communities but will also alter sediment biogeochemistry and, possibly, the energy transfer towards higher trophic levels. I also tested in mesocosm the possible use of the Mediterranean sea cucumber Holothuria tubulosa (Gmelin, 1788) as a bioreactor to counteract benthic eutrophication under different scenarios of sea warming due to CC. I investigated changes in OM features in oligo-mesotrophic and meso-eutrophic sediments and in feces of H. tubulosa under different temperatures, either after specimens’ acclimation (at 14, 17, 20, 23, 26, 29 °C) or after heat snaps (at 26 and 29 °C). The results suggest that H. tubulosa could be used to mitigate benthic eutrophication under different CC scenarios, but I also pinpoint that further studies are required to clarify if this species can adapt in the long term to persistent and more frequent MHWs. Moreover, I investigated the effects of intense rainfall and freshwater flash-flood events, among the major threats for coastal ecosystems worldwide, which are becoming increasingly frequent and catastrophic due to current CC especially in the Mediterranean Basin. I conducted two experiments, one in the field following natural rainfall events, and one in mesocosm simulating a major flooding on lagoon sediments, both aimed at investigating the effects of such events on sediments biogeochemistry of a river and a coastal lagoon. The results of such experiments pinpoint that these events can have differential consequences on sedimentary OM features with consequences that can last also during the entire recovery of natural salinity values. Finally, I carried out a metanalysis aimed at identifying the strongest effect of each selected disturbance on the tested variables and the most threatening disturbance (among those investigated in my thesis) on sedimentary OM stocks and degradation rates.