High metabolic rates in beach cast communities

Metabolic hotspots at land-water interfaces are important in supporting biogeochemical processes. Here we confirm the generality of land-aquatic interfaces as biogeochemical hot spots by extending this concept to marine beach cast materials. In situ atmospheric pCO_2, from a respiration chamber (10 cm in diameter and 20 cm high) inserted into wrack deposits, was determined using a high-precision (±1 ppm) non-dispersive infrared gas analyzer (EGM-4, PP-systems) at 1 minute recording intervals. The wrack deposits supported high metabolic activities, with CO₂ fluxes averaging (±SE) 6.62 ± 0.88 μmol C m^-2 s^-1, compared to median value of 0.98 μmol C m^-2 s^-1 (mean 2.21 ± 1.25 μmol C m^-2 s^-1) for bare sand adjacent to deposits. Wrack metabolic rates ranged 40-fold across beaches, from a minimum of 0.57 ± 0.22 μmol C m^-2 s^-1 to a maximum of 20.8 ± 5.04 μmol C m^-2 s^-1, both derived from beaches with deposits dominated by Sargassum. Rates tended to increase significantly (F test, P < 0.05) from the shoreline to reach maximum rates at about 10 m from the shoreline, declining sharply further from the shoreline, and increased with increasing thickness of the deposits (maximum about 10 cm deep), declining for thicker deposits. Wrack differing in composition had similar metabolic rates, although deposits consisting of a mixture of seagrass and algae tended to show somewhat higher rates. Our results show a meter square of wrack deposit supports a metabolic rate equivalent to that supported by 3 m² of living seagrass or macroalgal habitat. In wrack, the marine environment provides organic material and moisture and the land environment provides oxygen to render wrack ecosystems an efficient metabolic reactor. Intense wrack metabolism should also be conducive to organismal growth by supporting the development of a cryptic, but diverse wrack-based food web.