Thornber, Carol S
macroalgae; allelopathy; Ulva; algae blooms; co-culturing; diversity
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Several species of Ulva are commonly found in the waters of Narragansett Bay, especially in eutrophic waters, where they can form fast growing blooms that can have ecological and economic consequences. The formation and release of allelopathic chemicals has been previously documented in some species of Ulva, including Ulva lactuca. Three species of blade-forming Ulva are commonly found in Narragansett Bay and coastal Rhode Island, namely, U. compressa, U. lactuca, and U. rigida. We aimed to determine if these three species of Ulva had allelopathic effects by testing their impacts on the growth of other macroalgae. Cystoclonium purpureum, Chondrus crispus, and Ceramium virgatum tips were grown alone (control) or with either U. compressa, U. rigida, or U. lactuca in mesocosms separated by mesh for eight days, in two separate trials. The blotted-dry fresh weight of Cystoclonium, Chondrus, and Ceramium was recorded every other day and tips were photographed for surface area analysis. Nutrients were checked daily using NO3 as a proxy and adjusted to prevent nutrient limitation. All three species of Ulva had a significant negative effect on the growth of Cystoclonium, Chondrus, and Ceramium, although the effect was dependent on time. In the Cystoclonium trial, U. compressa and U. rigida treatments had the largest negative effect on Cystoclonium growth with overall mass loss observed after 6 days of co-culture. U. lactuca had a smaller negative effect on growth, with the average Cystoclonium growth rate after 8 days of co-culture (2.9 ± 0.9 % day-1) significantly below controls (5.0 ± 0.8 % day-1). In the Chondrus trial, there was a striking effect of all three Ulva species on the growth of Chondrus after only two days of co-culture. Growth rates of tips with all species of Ulva were <1% day-1 for the duration of the experiment, while the control tips increased in growth over time from 1.83 (± 0.7) % day-1 on Day 2 to 5.56 (± 0.9) % day-1 on Day 8. Overall mass loss was observed after 8 days of co-culture with U. compressa. The Ceramium growth was more variable over the course of the trial, but clear separation was seen on day 8 between the controls and the decreased growth of the tips co-cultured with Ulva. All three species of Ulva had a similar effect on the growth of Ceramium at the end of the trial. Our results indicate that U. compressa, U. lactuca, and U. rigida can significantly inhibit the growth of other macroalgae. This has implications for secondary effects of Ulva blooms reducing the algal diversity in addition to primary effects of eutrophication.