The importance of recurrent reproductive events for Ruppia maritima seed bank viability in a highly variable estuary

Abstract

Submerged aquatic vegetation (SAV) that recruits from seed may be favored over species that maintain populations through clonal reproduction as estuaries become more dynamic under upstream hydrologic changes and sea level rise. We examined recruitment in an SAV species Ruppia maritima (wigeongrass) at the hydrologically variable Everglades-Florida Bay ecotone. We hypothesized that the R. maritima seed bank depends upon large reproductive events occurring at least annually. Vegetation biomass, nutrient allocation, sexual reproduction, seed set and sediment seed bank viability were examined when reproductive meadows were present and following senescence. Within a meadow, total seed bank densities were high (>20,000m−2). However, 85% of seeds had germinated, leaving a small persistent seed bank, and most germinations did not successfully produce seedlings. Only 25% of intact (potentially viable) seeds were viable (<768 seedsm−2) to create a persistent seed bank. As reproductive shoots senesced, all vegetation completely died and 2750seedsm−2 entered the seed bank (40seedsg−1 biomass), increasing total seed bank viability from <4 to ∼20%. Reproductive R. maritima meadows can produce numerous seeds if not nutrient-limited during “windows of opportunity”; however, lack of a persistent seed bank, high germination rate and total vegetation mortality following reproduction indicate a reliance on reproductive events for population regeneration. Under lower hydrologic variability, perennial populations of R. maritima and other SAV could be sustained through vegetative reproduction, thereby reducing dependence on sexual reproduction. Otherwise, SAV in variable environments like this ecotone will remain dependent recurrent reproductive events to sustain recruitment and vegetation maintenance.