Abstract
We examined the water quality benefits of V. americana, a submerged aquatic plant in the family Hydrocharitaceae, with a factorial manipulation of plant presence and nutrient enrichment in aquatic mesocosms and with in-situ experiments in Florida’s Caloosahatchee River Estuary (CRE). Nutrient addition caused severe algal blooms in the mesocosm tanks, but final chlorophyll-a levels trended lower in treatments vegetated with V. americana. Plant tissue data suggested that increased nutrient uptake by the submerged aquatic vegetation (SAV) occurred in enriched treatments, but SAV biomass was too low for this uptake to significantly reduce water column nutrient concentrations. Therefore, SAV-algae allelopathic interactions were a more plausible mechanism for the observed reduction in chlorophyll a in these mesocosm conditions. In the field, nutrient enrichment had no observable effects on plant characteristics, nor plants on water column nutrient concentrations. Plant tissue stoichiometry in the field was characterized by low C:N ratios in both enriched and ambient treatments, suggesting that the environment was already nutrient replete. Biomass and abundance data from field studies in the Caloosahatchee Estuary were combined with tissue nutrient data from these experiments to estimate estuary wide nutrient sequestration by V. americana meadows. These calculations indicated that only a small amount of nutrients are currently being stored by the SAV standing stock 56.96 kg C, 5.12 kg N & 0.31 kg P, as result of low abundance and biomass. Yet, with increased restoration, V. americana stands to offer a substantial sink for macronutrient loads by sequestering nutrients in both plant material 191,507 kg C, 3,796 kg N & 1,081 kg P and in meadow associated sediments, 399,330 kg C y-1 & 39,933 kg N y-1. These findings emphasize the value and urgency of restoring V. americana in the CRE, a southwest Florida estuary plagued with a history of high nutrient concentrations and frequent algal blooms.
