Abstract
Aggregations of rhodoliths, habitat-forming, free-living coralline red algae, form beds throughout the world’s oceans. On Santa Catalina Island, California, USA, rhodolith beds occur in protected coves where dense networks of moorings support recreational boating activities. The chains and spreader lines associated with these moorings chronically disturb the benthos, crushing the rhodoliths and reducing biodiversity of rhodolith-associated communities. Here, we examine how mooring disturbance affects rhodolith photosynthesis and respiration and characterize rhodolith-associated invertebrate respiration to better understand how this disturbance affects productivity by the ecosystem. To do this, we used a respiration chamber in the laboratory to measure the amount of oxygen produced and/or consumed by undisturbed (intact) rhodoliths, mooring-disturbed (“crushed”) rhodolith fragments, and laboratory-crushed rhodoliths, and the amount of oxygen consumed by the dominant rhodolith-associated invertebrate taxa. Our results indicate that rhodolith maximum net productivity is significantly reduced and rhodolith respiration is significantly increased by mooring disturbance in the field, but that crushing of the rhodolith thalli alone does not result in immediate changes to either of these measures. Rather, it appears that chronic crushing of rhodolith thalli, which results in their mortality and rhodolith habitat degradation, is required to elicit these metabolic changes. In addition, we observed variation in respiration rates among the 5 most commonly observed invertebrate taxa within the Catalina Island rhodolith beds, and scaling these respiration rates by each species’ abundance in the rhodolith beds and in adjacent mooring-disturbed (degraded) habitats indicated that mooring disturbance results in a decrease in community respiration by approximately 2.61 mg O2·m–2·d–1, with individual species contributing between 0.05 mg O2·m–2·d–1 and 1.84 mg O2·m–2·d–1 to this decrease. This study provides insight into the consequences of anthropogenic disturbance on productivity and respiration in these ecologically important habitats.
Highlights
Physical disturbances are important in structuring natural ecosystems (Hobbs and Huenneke 1992, Worm et al 2006, Pickett and White 2013)
Net productivity was significantly lower in the mooring-disturbed Crushed rhodolith fragments (CRF) than in the undisturbed rhodoliths (Fisher’s LDS: P = 0.001) and in the laboratory-crushed rhodoliths (P = 0.048), but it did not differ between the undisturbed rhodoliths and the laboratory-crushed rhodoliths (P = 0.129) (Fig. 2)
Our results show that the chronic crushing of rhodolith thalli by boat mooring chains alters patterns of rhodolith metabolism
Summary
Physical disturbances are important in structuring natural ecosystems (Hobbs and Huenneke 1992, Worm et al 2006, Pickett and White 2013). They can result in increased mortality, changes to the physical environment, altered food web structure, and reduced ecosystem functioning. The loss of a foundation species can result in altered ecosystem function (Ellison et al 2005), which includes controlling rates of primary productivity by the autotrophs, and respiration by the autotrophs, heterotrophs and microbes (del Giorgio et al 1997, Duarte and Agustí 1998, Edwards et al 2020). In the coastal marine environment, conspicuous foundation species create coral reefs (Luckhurst and Luckhurst 1978), mangrove habitats (Nagelkerken et al 2008), kelp forests (Graham 2004, Graham et al 2007), seagrass beds (Orth et al 1984), and rhodolith beds (Graham et al 2016, Tompkins and Steller 2016, Gabara et al 2018, McConnico et al 2018)
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