Abstract
Blooms caused by the green macroalga Ulva pose a serious threat to coastal ecosystems around the world. Despite numerous studies of the causes and consequences of these blooms, we still have a limited understanding of Ulva bloom species richness and abundance due to difficulties in identifying Ulva species using morphological features. Along the northeastern U.S. coastline, all blooms of distromatic Ulva blades were previously identified as Ulva lactuca. Recent molecular sequencing, however, discovered the presence of additional distromatic Ulva species. Therefore, in order to determine the relative abundance of Ulva species within blooms, we conducted monthly surveys at four Narragansett Bay, RI, sites representing a gradient of bloom severity. We found that the biomass of Ulva within blooms was a mix of Ulva compressa and Ulva rigida, not U. lactuca as previously reported. In contrast, sites not impacted by blooms that were located near the mouth of Narragansett Bay were dominated by U. lactuca. We also observed spatial and temporal differences in Ulva and total macroalgal diversity between bloom-impacted sites, indicating that Ulva bloom composition can be radically different between similar sites within close proximity. We discuss our results in the context of Ulva blooms worldwide, highlighting the need to definitively determine bloom species composition in order to fully understand bloom dynamics.
Highlights
The formation of blooms of filamentous and/or thin foliose macroalgae are frequently a consequence of coastal eutrophication (Fletcher, 1996; Valiela et al, 1997; Morand and Merceron, 2005; Ye et al, 2011)
While tubular Ulva species in Rhode Island cannot be identified to the species level based on morphological features alone, significant progress has been made in distinguishing between Ulva blades in this region
Ulva lactuca dominated at Graduate School of Oceanography (GSO), where we only found small fragments (< 0.4 g per piece) of U. compressa and U. rigida during three of the seven survey months (Figure 2b)
Summary
The formation of blooms of filamentous and/or thin foliose macroalgae are frequently a consequence of coastal eutrophication (Fletcher, 1996; Valiela et al, 1997; Morand and Merceron, 2005; Ye et al, 2011). Bloom macroalgae often form large floating mats in the water column, in which individual thalli grow, fragment, and asexually reproduce via zoospores (Gao et al, 2010; Ye et al, 2011) These floating mats of algae alter coastal light, nutrient, and water flow conditions, causing decreases in perennial algae, seagrasses, and benthic invertebrates (Valiela et al, 1997; Hauxwell et al, 1998, 2001; Thomsen and McGlathery, 2006; Worm and Lotze, 2006). Several species produce toxins that negatively impact co-occurring organisms
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