Drift macroalgae positively influence seagrass-associated nekton communities of the northern Gulf of Mexico

Abstract

Drift macroalgae, often found in clumps or mats adjacent to or within seagrass beds, can provide additional food resources and habitat complexity, leading to increased animal abundance, but large concentrations can also inhibit faunal movements, smother benthic communities, and contribute to hypoxia, reducing nekton abundance. Despite its ubiquity, few studies have quantified drift macroalgal prevalence over large spatial scales or its effects on seagrass-associated nekton, hindering our understanding of the functional role of drift macroalgae in ecosystems. We quantified the relationship between drift macroalgal biomass and the seagrass-associated nekton community within five estuaries spanning 2000 km across the northern Gulf of Mexico. Overall, increases in macroalgal biomass within seagrass meadows significantly influenced community structure, increasing shrimp, crab, and fish abundances, but the effect varied by region. Relationships between species richness, diversity, organism size, and macroalgal biomass were not observed, suggesting that drift macroalgae provide additional habitat but not necessarily new niche space. Small nekton play a vital role in many local fisheries, providing valuable food resources for fish and invertebrates. Increased recruitment into macroalgae can benefit local fisheries by providing shelter and increased food resources, which may increase the survival, growth, and population size of recreationally and economically important species. While excess levels of drift macroalgae can negatively impact benthic plant and animal communities, particularly in eutrophic areas, the moderate levels observed during this survey were associated with positive effects on organismal abundance, suggesting that drift algal dynamics should be considered in habitat-based management strategies for coastal estuaries.

Spatial Distribution and Potential Impact of Drifted Thalli of the Invasive Alga Rugulopteryx okamurae in Circalittoral and Bathyal Habitats of the Northern Strait of Gibraltar and the Alboran Sea

“…The degradation of these clumps on top of these small filter-feeding invertebrates may cause an increase in nutrients and hypoxia, together with a physical barrier for capturing food particles from the water column, causing a decline in their population [32]. Indeed, some authors have detected an increase in deposit feeders together with declines in filter feeders and photosynthetic organisms in areas with large clumps of drifted algae [32][33][34][35].…”

Seagrass-macroalgal interactions in a changing ocean

“…When not in excess, macroalgae coexist with seagrass meadows and provide support to community and ecosystem secondary productivity. For instance, drift algae in the Northern Gulf of Mexico have been found to positively influence seagrass-related nekton recruitment and biomass (Correia and Smee, 2022;Correia et al, 2022b). However, the recent IPCC Special Report on the Ocean and Cryosphere in Changing Climate (Bindoff et al, 2019) reported a widespread increase in coastal stress levels, noting that since 1980, shifts in seaweeds and harmful algal blooms, that often interact negatively with seagrass meadows, have also increased in range and frequency.…”