Feedback effects in a coastal canopy-forming submersed plant bed

Although physical and biogeochemical properties of an environment determine distribution and health of biota, some organisms modify habitat conditions through complex interactions with their surroundings. We quantified effects of the canopy-forming submersed plant species Stuckenia pectinata on local hydrodynamics and explored resulting positive and negative feedbacks on plant growth. Measurements of waves and tidal currents were made outside, at the edge of, and within a plant bed located in the mesohaline region of Chesapeake Bay. Clear feedback effects on light, nutrients, and sediments were observed, and were found to vary seasonally with plant growth cycle. During the June period of peak plant biomass, significant wave heights were attenuated by ~ 37% within the plant stand; this resulted in an ~ 60% reduction of total suspended solids, which was stable and relatively unaffected by periods of high wind speed or water depth. Deployments of artificial substrates showed that epiphytic accumulation was greatly reduced within the plant bed, further increasing available light for plants to 25% of incoming irradiance (as compared to 0.2% outside the plant bed). In addition, higher particle trapping rates and sediment organic content augmented bed pore-water nutrient pools (CO₂, NH₄⁺, PO³⁴⁻) sufficiently to satisfy plant demands. These processes generated negative feedback effects on plant growth, including elevated pore-water sulfide (> 700 μmol L⁻¹) and depressed water-column O₂ concentrations (< 2 mg L⁻¹), but levels were ephemeral and generally outside reported stress thresholds. Dominant positive feedbacks provide an explanation for bed survival in this environment despite degraded water quality during summer months.