Replacement of oyster reefs by mangroves: Unexpected climate‐driven ecosystem shifts

Abstract

Increases in minimum air temperatures have facilitated transitions of salt marshes to mangroves along coastlines in the southeastern United States. Numerous studies have documented mangrove expansion into salt marshes; however, a present‐day conversion of oyster reefs to mangrove islands has not been documented. Using aerial photographs and high‐resolution satellite imagery, we determined percent cover and number of mangrove patches on oyster reefs in Mosquito Lagoon, FL, USA over 74 years (1943–2017) by digitizing oyster reef and “mangrove on oyster reef” areas. Live oyster reefs present in 1943 were tracked through time and the mangrove area on every reef calculated for seven time periods. There was a 103% increase in mangrove cover on live oyster reefs from 1943 (6.6%) to 2017 (13.4%). Between 1943 and 1984, the cover remained consistent (~7%), while between 1984 and 2017, mangrove cover increased rapidly with a 6% year−1 increase in mangrove area on oyster reefs (198% increase). In 1943, 8.7% of individual reefs had at least one mangrove patch on them; by 2017, 21.8% of reefs did. Site visits found at least one mature Avicennia germinans on each tracked mangrove reef, with large numbers of smaller Rhizophora mangle, suggesting the post‐1984 mangrove increases were the result of increased R. mangle recruitment and survival. Escalation in the coverage and number of mangrove stands on oyster reefs coincided with a period that lacked extreme freeze events. The time since a temperature of ≤−6.6°C (A. germinans mortality threshold) and ≤−4°C (R. mangle mortality threshold) were significantly correlated with the increased ratio of mangrove area:oyster area, total mangrove area, and number of mangrove patches, with greater variation explained by time since ≤ −4°C. The lack of freezes could lead globally to an ecosystem shift of intertidal oyster reefs to mangrove islands near poleward mangrove range limits.

Some Like it Hot: The Ecology, Ecosystem Benefits and Restoration Potential of Oyster Reefs in Tropical Waters

“…For example, reef-building oysters require hard substrates for settlement, and tropical oysters can be found attached to rocks, mangrove roots and coral reefs, and many species rely on these habitats to form biogenic reef structures (Table 1). Driven by climate change, oyster reefs in parts of the subtropical southeast USA are being replaced by mangroves, primarily Rhizophora mangle, from tropical areas (Mcclenachan et al, 2021). Both the tropical Crassostrea rhizophorae and eurythermal Crassostrea virginica have been documented growing on the roots of R. mangle (C. rhizophorae exclusively so) (Aquino-Thomas and Proffitt, 2014; Lodeiros et al, 2020), however the preferred habitat of C. virginica is hard rocky substrates (Carranza et al, 2009).…”

Biodegradable Material for Oyster Reef Restoration: First-Year Performance and Biogeochemical Considerations in a Coastal Lagoon

“…Areal coverage of natural intertidal C. virginica reefs within ML declined 24% between 1943 and 2009 [3], primarily in popular boating areas due to boat wakes dislodging live oyster clusters and depositing shells above the intertidal zone where survival plus natural recolonization are not possible [4]. Oyster reefs situated in shallow bays and tidal creeks bordered by mangroves and salt marsh plants did not change in aerial coverage over this same time frame [37]. This loss of oyster reef habitat, coupled with declining water quality in ML, initiated a community-based restoration effort in 2007 that remains on-going.…”

“…This study, as well as previous studies in ML, have all demonstrated a strong "reef effect" for sediment biogeochemical parameters, meaning each individual reef tends to be highly unique, including those within the same treatment or category [20,40]. Causes for this high within-treatment variability include the unique site histories of each reef, differences in local flow patterns, proximity to nutrient or pollution sources, exposure to boat traffic or other stressors, spatial variation in disease or predation, and mangrove encroachment [4,37]. Therefore, the low number of replicate reefs, the high within-treatment variability, and the naturally heterogeneous nature of many of the sediment biogeochemical parameters measured, all contributed to low statistical power for the field BACI experiment.…”

Working With Nature Approaches for the Creation of Soft Intertidal Habitats

“…Similarly, mangroves can reduce sedimentation and salinity stress for other mangroves or other foundation species such as oysters (Aquino- Thomas and Proffitt, 2014). Oyster reefs can, in turn, provide new habitat for mangroves to colonize, and augment the settlement of mangrove propagules (McClenachan et al, 2021). As such, by fostering beneficial interactions between neighboring conspecifics, the dense restoration of these intertidal keystone species can provide improved restoration success.…”