Recreational clam harvesting affects sediment nutrient remineralization and the growth of the green macroalga Ulva lactuca

Recreational clam harvesters at Northwest Penn Cove leave pools in the sediments that fill with porewater before being inundated by the incoming tide. We hypothesized that this nutrient-rich porewater may affect ulvoid algal growth. To test this, we measured nutrient concentrations in seawater, tidepools, shellfishers' pools, and in porewater from the top 9 cm of the sediment. On average, sediment porewater was 4495% higher in ammonium, 772% higher in nitrite, and 3238% higher in phosphate than nearby seawater; nitrate concentrations in porewater were 35% of the seawater concentrations. Water in shellfishers' pools was 291% higher in ammonium and 122% higher in nitrite than seawater. Nitrate and phosphate concentrations in shellfishers' pools and seawater did not differ significantly. We then transplanted Ulva lactuca around shellfishers' pools, tidepools, and onto the mudflat. Transplanted algae did not differ significantly in tissue carbon, nitrogen, and phosphorus concentrations after 2 days. However, the algae transplanted around shellfishers' pools grew 20% more over two weeks than the algae transplanted around tidepools and onto mud flats. Our results suggest that the nutrient-rich water in shellfishers' pools enhances algal growth near the pools and may contribute to the formation and persistence of the Ulva bloom at this site.

Research highlights
► Intertidal sediments contained higher nutrient concentrations than nearby seawater.
► Clam harvesters dug pools into the sediments that filled with porewater.
► Water in clam harvesters' pools had higher nutrient levels than the nearby seawater.
► Ulva adjacent to clam harvesters' pools grew faster than Ulva not near the pools.
► C, N, and P in Ulva adjacent to and away from shellfishers' pools were not different.