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Coastal & Estuarine Science News (CESN)

Coastal and Estuarine Science News (CESN) is an electronic publication providing brief summaries of select articles from the journal Estuaries and Coasts that emphasize management applications of scientific findings. It is a free electronic newsletter delivered to subscribers on a bi-monthly basis.

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May 2022

Table of Contents

The Future of Oyster Aquaculture in Seagrass Beds

What changes will sea level rise bring?

Commercial oysters are commonly cultivated within the seagrass beds of U.S. west coast estuaries, where interactions between bivalves and seagrasses can be quite complicated. Although harvesting techniques can reduce seagrass density, oysters can also enhance seagrass productivity through sediment nutrient addition and filtering water that in turn increases light penetration. Natural resource managers and aquaculturists alike are trying to understand how this interaction will be affected by future shifts in seagrass distribution driven by sea level rise.

Researchers evaluated the relationships between the presence of native eelgrass (Zostera marina) and environmental conditions in Washington’s Willapa Bay, which produces up to 17% of the nation’s commercial oysters. Bathymetric elevation had the highest relative influence on eelgrass coverage, followed by distance to estuary mouth and sediment silt–clay percentage.

The model they developed was then used to predict the distribution of eelgrass in 458 aquaculture beds in the bay for the years 2030, 2050, and 2100 under both conservative and high rates of sea level rise. Their analysis suggests that Z. marina coverage will increase for all SLR scenarios and years—resulting in as much as 34% more eelgrass in the estuary and a 40% increase within aquaculture beds by 2100.

The predicted increase in eelgrass coverage could present multiple challenges for oyster growers and estuarine managers as they work to protect both resources. Not only would eelgrass expansion potentially impact shellfish growth and harvest, but increasingly stringent regulations may also restrict areas where oyster aquaculture is permitted. Future management strategies should consider mutual benefits where overlap occurs, especially when seagrass is not directly threatened and may even be enhanced by the presence of oysters. Protecting aquaculture operations and seagrass beds do not have to be mutually exclusive goals.

Source: Dumbauld, B.R. et al. 2022. Predicted Changes in Seagrass Cover and Distribution in the Face of Sea Level Rise: Implications for Bivalve Aquaculture in a US West Coast Estuary. Estuaries and Coasts. DOI: 10.1007/s12237-022-01060-2

The Case for Microbes in Coastal Restoration

Microbial symbionts are important for plant health

While much is known about plant and wildlife communities in coastal areas, the importance of microorganisms represents a large knowledge gap. Understanding the symbiotic relationship between plants and microbes—and then incorporating microbes when restoring coastal vegetation communities—may help improve restoration outcomes.

Researchers in New Orleans reviewed literature on the ecology of plant–­­microbial symbioses in coastal systems, including mycorrhizae, nitrogen fixers, endophytes, rhizosphere microbes, and pathogens, and their implications for restoration. They focused on four common coastal communities: sand dunes, marshes, mangroves, and forests or shrublands. They found that microbes influence plant establishment, growth, competitive ability, and stress tolerance, and also modulate biogeochemical cycling. Some common benefits of microbial symbionts relevant to restoration include assisting plants with nutrient uptake, conferring drought and salinity tolerance, producing plant growth hormones, enhancing plant stress tolerance, and protecting plants from pathogens.

The potential for microbes to enhance restoration projects depends on the reliance of plants on microbes, the type of inoculum used, and the type of restoration employed. Degraded, previously invaded, or restored sites far from natural areas may especially benefit from microbial inoculation, as long as the plants are adapted to them. Inocula can be sourced from whole soil or roots collected from a reference ecosystem or cultured and multiplied in a greenhouse or lab. Although generic commercial inocula are readily available, locally sourced microbes are a better source of mutualistic partners. Additionally, it’s easier to introduce microbes by outplanting live plants or saplings (rather than by seeding) because symbiosis can be established ahead of time in the greenhouse.

While still in its early stages, using microbial symbionts in coastal restoration is promising and presents many collaborative opportunities for restoration practitioners and microbial ecologists to work toward a common goal of enhancing the resilience of coastal ecosystems.

Source: Farrer, E.C. et al. 2022. Plant‑Microbial Symbioses in Coastal Systems: Their Ecological Importance and Role in Coastal Restoration. Estuaries and Coasts. DOI: 10.1007/s12237-022-01052-2

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Surf’s Up: How to Increase Diversity in Surf Zones

The importance of habitat complexity and connectivity for beaches

Sandy beaches dominate coastlines around the world, but surf zones are generally thought to have low structural complexity and therefore low diversity of fish species. However, linkages between where the waves break on the shore and nearby structurally complex or highly productive ecosystems suggest that the diversity and abundance of fish in surf zones may be higher than what’s expected.

To examine how proximity to adjacent habitats can shape the species richness and functional diversity within surf zones, researchers used baited remote underwater video systems to sample fish assemblages at 25 ocean beaches spanning 50 kilometers of coastline in southeast Queensland, Australia. These beaches differed in their proximity to the nearest rocky reefs, rocky headlands, and estuarine inlets, and in their extent of urbanization. The fish were grouped into seven categories based on feeding mode: zoobenthivore, zooplanktivore, piscivore, omnivore, corallivore, herbivore, and detritivore.

According to their findings, distance to rocky reefs was the key feature shaping surf communities. Beach systems with surf zones within 600 meters of rocky reefs supported fish assemblages with more species and a greater diversity of functional groups, as measured by the number of feeding categories. Species richness and functional diversity were also higher at offshore bars where waves first break compared to nearshore troughs. The effects of proximity to headlands, estuaries, or urbanized areas in this study area were not as important. 

Surf zones can be important conduits between reefs, estuaries, and offshore waters. Of the 70 species observed in this study, only 14 were found exclusively in surf zones with rocky reefs directly under the waves, suggesting a large diversity of species utilize a system that is traditionally viewed as having low habitat heterogeneity. These findings point to the importance of maintaining connectivity among different habitats to foster the greatest diversity of species and functions.

Source: Henderson, C.J. et al. 2022. Connectivity Shapes Functional Diversity and Maintains Complementarity in Surf Zones on Exposed Coasts. Estuaries and Coasts. DOI: 10.1007/s12237-022-01046-0

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Tropical Tourism Alters Seagrass Stable Isotope Compositions

Tracing nutrients in the world’s second-largest coral reef system

Among the most threatened marine ecosystems are those associated with tropical coasts. Belize’s Ambergris Caye Lagoon is adjacent to multiple protected areas within and around the Belize Barrier Reef Reserve System, a UNESCO World Heritage Site. However, infrastructure developed to support tourism has reduced terrestrial, fringing wetland, and aquatic vegetated habitat while increasing impervious surfaces. A team of researchers wanted to test whether the stable isotope composition of seagrass can identify spatial patterns in nitrogen loading from human development. They used impervious surface cover as a proxy for human land-use intensity and Thalassia testudinum turtle grass as a sentinel for tracing nutrient inputs and movements.

During March of 2017 and 2018, they collected T. testudinum samples from a 24-kilometer section of the seaward lagoon of Ambergris Caye. The sample sites were distributed across an inshore-offshore gradient and a south-to-north alongshore gradient. The team analyzed the carbon, nitrogen, and stable isotope composition of the leaf blades, as well as the epiphyte load, which is known to increase with eutrophication.

As expected, proximity to urban areas altered the nutrient composition of seagrass in the lagoon. Seagrass δ15N was highest adjacent to or slightly downcurrent of the urban center of San Pedro, and C:N ratios were reduced (indicating more N). Epiphyte loads also increased with proximity to the most urbanized areas. Although the effects are strongest near San Pedro and outlying developments, spatial patterns in seagrass δ15N suggest that even marine protected areas farther away are influenced by nutrient loading.

Seagrass tissue stable isotope composition analysis is a low-cost means of tracing nutrient inputs, and spatially explicit information will be vital in helping natural resource managers and land-use planners address eutrophication in vulnerable tropical coastal habitats.

Source: Murphy, T.E. et al. 2022. Seagrass Stable Isotope Composition Provides SeascapeScale Tracking of Anthropogenic Nitrogen Inputs in a Tropical Marine Lagoon. Estuaries and Coasts. DOI: 10.1007/s12237-022-01058-w

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