Carbon and nutrient fluxes from seagrass and mangrove wrack are mediated by soil interactions

Abstract

Shoreline accumulation of vegetative wrack may contribute nutrient subsidies for primary or secondary production, detritivore food-web support, habitat provision, and sediment stabilization. This study addresses a knowledge gap in the literature by going beyond quantification of carbon (C) and nutrient flux potentials to investigate the biogeochemical interaction between wrack and shoreline soil. Results from a year-long shoreline survey of wrack type and abundance in Mosquito Lagoon, FL, were combined with a two-month intact core experiment observing the interaction of mangrove and seagrass leaves with organic and mineral shoreline soils. Experimental treatments were subjected to tidal cycles; at low tide, measurements were made of soil respiration and drainage water nutrient content. A measure called Soil Interaction with Litter Effect (SoILE) is introduced to quantify the suppression or enhancement of C and nutrient fluxes from a combination of wrack with shoreline soils compared to the potential fluxes of wrack and soil separately.Monthly mean (±1 S.E.) accumulation of wrack dry mass in Mosquito Lagoon was 37.65 ± 2.99 g m−2. Unusually high values (72.16 ± 13.74 g m−2) occurred in September 2017 following Hurricane Irma. In a comparison of seagrass and mangrove leaves and organic and mineral soils, seagrass leaves were the greatest source of dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (SRP) by factors of 24 and 13, respectively. Seagrass also had the highest fluxes of dissolved organic carbon (DOC) and CO2 by factors of 2 and 2.7, respectively. The most common effect of combining wrack and soil was to suppress the magnitude of DOC and CO2 fluxes. There were few differences between combined and additive fluxes of DIN and SRP; the DIN flux was suppressed from the combination of seagrass with organic soil, and the SRP flux was enhanced from the combination of mangrove leaves with mineral soil. Suppression and enhancement of SoILE values were likely both attributable to a “smothering” effect caused by the physical interaction of vegetation with the soil surface that altered redox conditions. Wrack accumulation on restored living shorelines is likely to have a two-fold positive effect of subsidizing nutrients for primary production by shoreline plants and contributing to an increase in soil carbon by interacting with soils to result in a net suppression of DOC and CO2 effluxes.