Bloom-forming macroalgae (Ulva spp.) inhibit the growth of co-occurring macroalgae and decrease eastern oyster larval survival

Macroalgal blooms have increased in frequency worldwide due to anthropogenic activities. Algal blooms can disrupt recreational activities, interfere with fisheries, and deplete oxygen during decomposition. Narragansett Bay has experienced macroalgal blooms dominated by blade-forming macroalgae of the genus Ulva for over a century. Evidence from other systems has suggested that Lava can negatively impact other organisms. The first objective of this study was to determine whether bloom-forming U. compressa and U. rigida inhibit the growth of co-occurring macroalgae - Gracilaria vermiculophylla, Cystoclonium purpureum, and Chondrus crispus -during co-culture via laboratory based assays. We found that U. compressa and U. rigida significantly inhibited the growth of all 3 macroalgae. We were able to verify the negative effect of U. compressa, but not U. rigida, on the growth of G. vermiculophylla in flow-through seawater tanks. Our second objective was to determine if Ulva exudate decreased the survival of eastern oyster larvae in laboratory challenge experiments. We documented a significant negative effect of Ulva exudate on oyster survival, which depended on both the Ulva species and the nutrient condition. The strongest effect on oyster larval survival was seen in larvae exposed to nutrient-replete U. compressa exudate, which had <30% relative survival after 1 wk. Our results indicate that bloom-forming Ulva has the potential to inhibit co-occurring macroalgae and cause oyster larval mortality.