Bioinvasion by Spartina Patens Alters Sediment Biogeochemical Functioning European Salt Marshes

Abstract Bioinvasions pose undeniable threats and trigger changes in salt marsh ecosystem functioning. In Mediterranean and Atlantic marshes, the invasion by S. patens contributed to added competitive pressure to endemic middle-upper marsh species such as H. portulacoides. The introduction of a new and aggressive non-indigenous species (NIS) is a game-changer for the marsh biogeochemical functioning. In the present study we explored effects of this bioinvasion on an array of different extracellular enzymatic activities (EEA) of salt marsh sediments colonized by NIS and endemic plant species and its impacts on the biogeochemical functioning of a large estuarine ecosystem. In H. portulacoides there was an evident control of the extracellular enzymatic activities (EEA) by the sediment physic-chemical traits, but these relationships were weaker and fewer in number when compared to the EEA and the sediment abiotic traits in the sediments colonized by S. patens. This indicates a prevalence of a plant effect over abiotic traits in controlling EEA. Disruption in biogeochemical functioning was evident in the sediments colonized by S. patens, with clear enzymatic activity peaks during spring and summer and with significantly higher values of several EEA during autumn and winter. These led to an acceleration of necromass decomposition processes and thus to reduced marsh storage and remediation capacity. Spartina patens also showed increased phosphatase, urease, and protease activity contributing to increased release of the inorganic phosphorous and nitrogen (ammonia) into the system, with possible consequences for its eutrophication status. Higher activity of carbon-based substrates decomposition allied to an increase in dehydrogenase activity (proxy to microbial abundance and respiration), will lead to an inevitable decrease in the carbon sink capacity of the sediments and reduction of the blue carbon storage ecosystem service. The sulfatase activity in the S. patens rhizosediments also showed a significant decrease during autumn, leading to reduced production of sulfate, sulfides and increased metal bioavailability. All these changes amount to a loss of the sink capacity of the system, in terms of eutrophication reduction through organic nitrogen and phosphorous retention, carbon sink due to the acceleration of the decomposition processes and decreased metal remediation capacity. Thus, the present work highlights the need to control this invasive species not only to prevent potential reductions in floristic diversity but also to prevent the loss of key biogeochemical services, that can impact the entire estuarine system.