Rhodolith structural loss decreases abundance, diversity, and stability of benthic communities at Santa Catalina Island, CA

Abstract

Species that increase habitat structural complexity often have a disproportionate influ ence on their ecosystems. Rhodoliths are bed-forming unattached coralline algae which in crease benthic structural complexity and enhance biodiversity in coastal soft-bottomed ecosystems worldwide. Consequently, their degradation due to anthropogenic disturbance, such as crushing from boat mooring chains, may lead to reduced biodiversity and ecosystem functioning. To examine how anthropogenic disturbance impacts rhodolith community dynamics, we used a comprehensive sampling and analytical approach to compare macroalgal, invertebrate (infauna and epifauna), and fish assemblages between rhodolith beds and adjacent mooring-disturbed crushed rhodolith sand. Sampling was conducted during 2 sampling times across 3 sites at Catalina Island, CA, USA. Our results demonstrate that the more heterogeneous structure provided in less disturbed rhodolith beds supported greater community richness and abundances than crushed rhodolith sands. Specifically, disturbance-related rhodolith structural loss was associated with significantly reduced richness of invertebrates and abundance of macroalgae, invertebrates, and fish. In particular, depositfeeding infaunal tanaids were far more abundant in rhodolith beds and drove much of the difference in invertebrate abundance between habitats. Further, spatiotemporal variation in the infaunal invertebrate assemblages was 54% lower in the rhodolith beds than crushed rhodolith sand, suggesting that rhodolith beds support more stable communities. Our results suggest that structured rhodolith bed habitats support more abundant, diverse, and stable benthic communities than do disturbed rhodolith sand habitats. Better management of rhodolith ecosystems and the factors that disturb them could help maintain coastal biodiversity and stability.

Bottom Trawling Threatens Future Climate Refugia of Rhodoliths Globally

“…Additionally, as rhodolith genetic and species diversity are still greatly underestimated (Twist et al, 2019;Caragnano et al, 2020), putative diversity losses may occur undetected, leading to potentially misperceived assessments of the rhodolith biome diversity and phylogeography, and gene pool conservation status (i.e., shifting genetic baselines; Assis et al, 2014). The exact ramifications of climate change on rhodolith ecosystems are hard to predict, as new insights on their role as bio-engineers and their complex relationships with associated species are only starting to be discovered (Steller et al, 2003;Horta et al, 2016;Gabara et al, 2018;Fredericq et al, 2019;Carvalho et al, 2020). For example, recent insights indicate that rhodoliths may be functioning as banks of microscopic stages (equivalent to seedbanks) for marine biodiversity, safeguarding species after local environmental stress incidents, as in the case of the 2010 Deepwater Horizon Oil Spill (Fredericq et al, 2019).…”

“…For example, recent insights indicate that rhodoliths may be functioning as banks of microscopic stages (equivalent to seedbanks) for marine biodiversity, safeguarding species after local environmental stress incidents, as in the case of the 2010 Deepwater Horizon Oil Spill (Fredericq et al, 2019). As climate change is threatening the ecological balance of the complex and dynamic relationships shaped within the rhodolith ecosystems, potential cascading effects caused by species turnover (e.g., predominance of competitive fleshy algae) may lead to substantial biodiversity and biomass reduction of their entire ecosystem (Horta et al, 2016;Legrand et al, 2017;Gabara et al, 2018).…”

“…Combining distributional predictions through time yielded an estimate of climate refugia, i.e., key regions presenting suitable conditions for rhodoliths to thrive in both the present and future (Morelli et al, 2020). As rhodoliths support and promote high levels of biodiversity (Teichert, 2014;Gabara et al, 2018;Fredericq et al, 2019), their refugia may become important biodiversity hotspots and, therefore, high-priority conservation regions (Hoegh-Guldberg and Bruno, 2010;Groves et al, 2012). Such refugia were mainly predicted to persist in cold-temperate regions of the world, namely northern Mediterranean, northeast Atlantic and Pacific and south America (Figures 3, 4).…”

“…Rhodolith refugia are under great local threat from intensive bottom trawling (∼79,000-114,000 days annually), requiring urgent conservation management measures and regulation of this activity. Bottom trawling causes massive mortality of rhodoliths and their associated organisms, as it crushes, buries and eliminates any natural bottom features, while the plume of sediments lifted by trawling reduces light availability, blocks photosynthesis and contributes to additional losses of nearby, non-trawled rhodoliths (Hall-Spencer and Moore, 2000;Thrush and Dayton, 2002;Steller et al, 2003;Wilson et al, 2004;Gabara et al, 2018;Bernard et al, 2019). Such profound impacts are mostly irreversible and damages can still be observed for years post-trawling (Hall-Spencer and Moore, 2000; but see Barberá et al, 2017;Ordines et al, 2017).…”

Variation in Photosynthetic Performance Relative to Thallus Microhabitat Heterogeneity in Lithothamnion australe (Rhodophyta, Corallinales) Rhodoliths

“…2006, Gabara et al. 2018) and support complex microbial communities (Cavalcanti et al. 2018), it is important to consider these microhabitats in addition to physical disturbance when describing their thallus‐specific productivity under different environmental conditions.…”