This project explored the feasibility of Eastern oyster (Crassostrea virginica) and sea lettuce (Ulva spp.) co-culture with a field experiment deployed at a shellfish farm in Virginia. We examined how relative oyster density (empty, low, high – with high density at 700 oysters and normal density at 400 initially, then divided in half for each subsequent split (decreasing density of oysters in bags for biological reasons)) fully crossed with relative Ulva density (empty (0 g), low (150 g), high (300 g)) in Zapco™ floating baskets affects 1. dissolved oxygen (DO), chlorophyll-a, and pH, 2. oyster performance, and 3. Ulva performance, all within oyster grow-out containers.
Twice per month from May - October 2024, diurnal water samples were collected from the inside of floating oyster grow-out bags using a peristaltic pump, and water parameters were assessed in real-time with a sonde. Oyster samples were collected periodically from each bag (n=15 per bag) and processed in a laboratory to determine if performance varied among treatment groups. Once per month, Ulva was removed from individual bags, weighed, and redeployed at the high or low Ulva stocking density threshold + 10 g.
Water: In a representative water sampling event on June 10, 2024, Ulva stocking density significantly affected DO and pH (p < 0.01 for all), while neither Ulva nor oyster stocking density affected chlorophyll-a levels within bags (p > 0.05). The presence of Ulva was associated with higher DO and pH levels. Oysters: Oyster height (measured from umbo to bill), dry tissue weight (DTW), and dry shell weight (DSW) were all significantly affected by oyster stocking density (p < 0.01), where the oysters were smaller in the higher density treatment. Oyster height was also significantly impacted by Ulva density, where the presence of Ulva was associated with smaller oysters. Ulva: In terms of overall production, Ulva yield from July – September was highest in the ‘normal’ Ulva stocking density treatments with either no oysters (130-185% increase from initial 150 g of Ulva) or ‘normal’ oyster stocking density (75-130% increase from initial 150 g of Ulva). High oysters/high Ulva produced the least amount of Ulva (2-20% increase from initial 300 g of Ulva).
Results suggest that Ulva can improve DO and pH conditions within oyster grow-out containers, but an oyster growth penalty is associated with this co-culture practice (potentially because Ulva may reduce water flow in the containers). Even with this growth penalty, Ulva could still be a useful tool for farmers experiencing the effects of climate change via ocean acidification and decreased DO related to rising water temperatures.