Phosphorus alleviation of salinity stress: effects of saltwater intrusion on an Everglades freshwater peat marsh

Abstract

Saltwater intrusion and salinization of coastal wetlands around the world are becoming a pressing issue due to sea level rise. Here, we assessed how a freshwater coastal wetland ecosystem responds to saltwater intrusion. In wetland mesocosms, we continuously exposed Cladium jamaicense Crantz (sawgrass) plants and their peat soil collected from a freshwater marsh to two factors associated with saltwater intrusion in karstic ecosystems: elevated loading of salinity and phosphorus (P) inputs. We took repeated measures using a 2 × 2 factorial experimental design (n = 6) with treatments composed of elevated salinity (~9 ppt), P loading (14.66 μmol P/d), or a combination of both. We measured changes in water physicochemistry, ecosystem productivity, and plant biomass change over two years to assess monthly and two‐year responses to saltwater intrusion. In the short‐term, plants exhibited positive growth responses with simulated saltwater intrusion (salinity + P), driven by increased P availability. Despite relatively high salinity levels for a freshwater marsh (~9 ppt), gross ecosystem productivity (GEP), net ecosystem productivity (NEP), and aboveground biomass were significantly higher in the elevated salinity + P treated monoliths compared to the freshwater controls. Salinity stress became evident after extended exposure. Although still higher than freshwater controls, GEP and NEP were significantly lower in the elevated salinity + P treatment than the +P treatment after two years. However, elevated salinity decreased live root biomass regardless of whether P was added. Our results suggest that saltwater intrusion into karstic freshwater wetlands may initially act as a subsidy by stimulating aboveground primary productivity of marsh plants. However, chronic exposure to elevated salinity results in plant stress, negatively impacting belowground peat soil structure and stability through a reduction in plant roots.

Periphyton as an indicator of saltwater intrusion into freshwater wetlands: insights from experimental manipulations

“…Because nitrite, nitrate, ammonium, TP, and SRP concentrations in fresh and salt source waters were not significantly different, our S treatment did not provide N or P subsidies Notes: Results of ANOSIM comparing recruitment community composition between all freshwater (F + FP) and all saltwater (S + SP) treatments and between all no P (F + S) and all + P (FP + SP) treatments are also reported. Wilson et al 2019). Declines in periphyton productivity with experimentally elevated salinity have also been reported from karstic wetlands in Belize (Rejm ankov a and Kom arkov a), however, the effects of salinity on periphyton ER were not reported in that publication.…”

“… , Wilson et al. ). Declines in periphyton productivity with experimentally elevated salinity have also been reported from karstic wetlands in Belize (Rejmánková and Komárková), however, the effects of salinity on periphyton ER were not reported in that publication.…”

“…, Wilson et al. ). Salinity was gradually increased starting in February 2015 until the surface water concentration was within the target range of 5–10 psu (April 2005).…”

Coastal Ecosystem Vulnerability and Sea Level Rise (SLR) in South Florida: A Mangrove Transition Projection

“…The process of peat loss has been documented in various coastal wetlands (e.g., De Laune et al, 1994;Cahoon et al, 2003;Day et al, 2011;Voss et al, 2013) and has been attributed to a number of factors including changes in microbial processes, increased sulfate reduction, inadequate plant production, root death, and vegetation damage (Stagg et al, 2017;Chambers et al, 2019). While recent research is beginning to develop a mechanistic understanding of peat collapse in the Everglades (e.g., Wilson et al, 2018;Servais et al, 2019;Wilson et al, 2019), uncertainties remain particularly in regards to how freshwater and oligohaline peat soils respond to salinization and the relevance of salinity-induced pore dilation for matrix deformation in peat soils.…”

Physical and Biogeochemical Characterization of a Tropical Karstic Marsh in the Yucatan Peninsula, Mexico.

“…Karstic landscapes and their associated wetlands have important socioeconomic values including maintenance of general hydrological balance and ood regulation, drinking water supply, and water for grazing animals or agriculture. Karstic aquifers and wetland remain a signi cant source of drinking water all over the world and may play an especially vital role in ensuring adequate water supplies for human communities in generally dry surface landscapes (Beltram 2016, Wilson et al 2019).…”