The Abundance and Persistence of Temperate and Tropical Seagrasses at Their Edge-of-Range in the Western Atlantic Ocean

Abstract

Species, including seagrasses, at their range limits are uniquely vulnerable to climate change. In the western Atlantic Ocean, the biogeographic transition zone between temperate and tropical ecosystems is recognized as one of several global hotspots where poleward-flowing western boundary currents are forecast to warm faster than the global average. In this region seagrass ecosystem services are primarily supplied by two species, Zostera marina, a temperate seagrass at its southern range limit and Halodule wrightii, a tropical seagrass at its northern limit. Water temperatures in the study location in Back Sound, North Carolina, USA have gradually increased the length of the stressful summer season for Z. marina (beginning after 3 consecutive days of daily mean water temperatures >23°C, ending after 3 consecutive days <25°C) from 84 days in 1962 to 156 days in 2019. The occurrence of extreme water temperatures also increased resulting in temperatures ≥30°C occurring more frequently in the last decade (2009-2019) than the previous 10 years. Biomass and aerial imagery collected periodically from 1981-2019 indicate that Z. marina biomass remained stable until 2008 but declined to 30-year low levels by 2019. Meadow area estimated from imagery collected during peak Z. marina biomass did not show a significant trend over time; however, lowest meadow area during the time series was recorded in 2019. Despite summer warming, H. wrightii biomass remained steady between 1979-2019 but did not replace Z. marina as the dominant species in the cooler months. We hypothesize that persistence of temperate Z. marina populations under stressful water temperatures is positively influenced by water clarity, life history, and meadow stability, due in part to the consistent presence of tropical H. wrightii maintaining meadow biomass and area. However, temperate species in edge-of-range tropicalized meadows, are still limited by physiological thresholds, and when these limits are exceeded, related declines in meadow biomass and area may not be fully replaced by tropical species immediately. Therefore, while tropicalization of seagrass meadows may result in greater resilience to abiotic stressors in the short-term, declines in biomass and area during the process of tropicalization may have significant impacts on meadow function.

Multiple paternity, fertilization success, and male quality: Mating system variation in the eelgrass, Zostera marina

“…Zostera marina meadows growing at the southern edge‐of‐range in the western North Atlantic experience a high degree of thermal stress and annual summer mortality (Bartenfelder et al., 2022 ; Thayer et al., 1984 ). As a result, recent shifts from a primarily perennial to a mixed‐annual life‐history strategy have been observed (Jarvis et al., 2012 ), along with resultant increases in flowering densities (Combs et al., 2021 ; Jarvis et al., 2012 ; Thayer et al., 1984 ) and genetic diversity (Allcock et al., 2022 ).…”

“…At its southern edge‐of‐range, Z. marina populations experience annual loss of biomass due to heat stress and are shifting primarily to a mixed‐annual life‐history strategy characterized by annual, complete loss of biomass, development of both vegetative and reproductive shoots, reestablishment from seeds alone, and seedlings that flower in their first year of growth (Allcock et al., 2022 ; Bartenfelder et al., 2022 ; Jarvis et al., 2012 ). Because they rely heavily on sexual reproduction, southern edge‐of‐range meadows may represent valuable genetic diversity hotspots among Z. marina populations (Diekmann & Serrão, 2012 ).…”

Tropical cyclone impacts on seagrass-associated fishes in a temperate-subtropical estuary

“…maritima ) that can quickly recolonize after disturbance as well as Z . marina that utilizes a mixed-annual reproductive strategy that may enhance seagrass resistance and recovery from physical stressors [ 85 – 87 ]. Given that our trawls were conducted entirely within the seagrass meadow, our surveys may be less sensitive to changes in overall seagrass area.…”

“…However, few linkages have been investigated or demonstrated between the AMO and fish populations in estuarine or coastal waters of the western Atlantic [ 100 ], and rigorous long-term surveys are needed to examine the interactive effects of climactic oscillations on disturbances and marine ecosystem structure. Our study presents a case study of coastal ecosystem response to cyclones at a unique biogeographic transition zone that may be particularly sensitive to climate-induced shifts in community structure [ 87 , 101 ] and may serve as a framework for future examinations of tropical cyclones under a global change scenario.…”

Ascidian diversity and abundance in North Carolina seagrass meadows

“…North Carolina (NC) coastal habitats are influenced by the convergence of warm waters that flow up from the Gulf Stream and cooler waters that originate from the Labrador Current moving south, creating a north–south temperature gradient (Coffey & Read, 2007; Olsen et al, 2004). Therefore, NC coastal waters are a biogeographic transition zone that can support species from both tropical and temperate regions (Bartenfelder et al, 2022; Thayer et al, 1984). In NC, seagrass meadows consist of three species (Short et al, 2007; Thayer et al, 1984): temperate Zostera marina (eelgrass), subtropical Halodule wrightii (shoal grass), and eurythermal Ruppia maritima (widgeon grass).…”