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Decoupling seasonal and temporal dynamics of macroalgal canopy cover in seagrass beds
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2020
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Source: Journal of Experimental Marine Biology and Ecology, 525, 151310
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Journal Title:Journal of Experimental Marine Biology and Ecology
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Description:Many seagrass habitats are susceptible to undesirable macroalgal overgrowth in response to growing watershed development, but the mechanisms leading to overgrowth remain unresolved. Partitioning the influences of intermittent seasonal cycles and directional human stressors is one main challenge. We examined the dynamics of macroalgal canopies within Halodule uninervis beds across a 10-year period in a tropical lagoon and first hypothesized that seasonal and temporal variances were distinguishable. We found that cooler and dryer winter months were consistently associated with blue-green algal canopies, mainly Lyngbya sp. and Phormidium sp., with an inverse relationship between sea-surface temperatures and coverage. For example, given that winter temperatures during the coolest month of February ranged between ±1.0 °C of the mean over the past decade, the expected coverage of blue-green algae shifted by ±15%. In contrast, the warmer and wetter summer months were associated with red algae canopies, predominantly Acanthophora spicifera, which were positively related to rainfall. Last, a weaker seasonal trend existed between some green and brown algae and a tidal-height proxy to groundwater discharge where karst watersheds existed. We next hypothesized that removing the variances associated with seasonal growth cycles in the differing phyla would reveal temporal trends in total macroalgal persistence associated with watershed development that were otherwise masked. Following seasonal adjustments, we found that persistent macroalgal canopies existed in the central, urbanized lagoon over the past decade; however, when moving to the north or south of the urban center, significant increases in macroalgal canopies were revealed. Watershed size and development were the strongest predictors of seasonally-adjusted macroalgal canopies through time. No similar trends were found when using unadjusted macroalgal cover data. More importantly, predictions were used to determine thresholds in watershed development associated with the transition between seagrass-versus-macroalgal dominance. The results offered a novel approach for ecological studies that may be limited by financial and logistical constraints to partition seasonal versus temporal change, and better appreciate their differences. For our study lagoon, the results unmasked a growing human footprint into seagrass beds and identified predictive thresholds for management to consider.
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Source:Journal of Experimental Marine Biology and Ecology, 525, 151310
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ISSN:0022-0981
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Rights Information:Accepted Manuscript
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