Oyster aquaculture has grown rapidly in recent years due to the declining wild populations. As the industry expands with the increased growth of the oyster aquaculture industry in the US, farmers consistently seek ways to optimize yields. Oyster farmers commonly make decisions about stocking density and control (or not) of biofouling with a focus on maximizing yield while reducing labor. These decisions can produce micro-environments that harbor conditions that differ substantially from the surrounding waters, influencing the growth and health of oysters. One key factor that can be impacted by these micro-environments is the availability of phytoplankton.
We investigated the effects of oyster stocking density and biofouling control on chlorophyll-a (chl-a) concentration and phytoplankton community composition in oyster grow-out bags at a commercial oyster farm located on the lower York River in Virginia. We hypothesized that increased oyster density and increased biofouling (resulting from a lack of air-drying measures taken to control biofouling) would decrease chl-a concentration and phytoplankton abundance, while creating a phytoplankton composition distinct from the surrounding water. The experimental design included three relative levels of oyster density (High, Normal, and Empty as a control) and two levels of biofouling (Fouled/Clean). Chl-a levels, a proxy for phytoplankton biomass, were measured across all six treatment combinations, each with three replicates. Additionally, a detailed phytoplankton abundance and community analysis was conducted for High Oyster Density/Fouled, High Oyster Density/Clean, and Empty of Oysters/Clean treatments. Water samples for chl-a and microscopy analysis were collected 13 times between September 2022 and June 2023.
Our results indicate that the High Oyster Density/Fouled treatment showed reduced phytoplankton abundance and chl-a concentration and altered phytoplankton species composition (but not in the High Oyster Density/Clean treatment). This reduction is attributed to increased phytoplankton consumption by more oysters combined with restricted water circulation due to biofouling. Biofouling control emerges as a critical strategy to ensure a consistent food supply for oysters and promote optimal growth.