Shorelines at the interface of marine, estuarine and terrestrial biomes are among the most degraded and threatened habitats in the coastal zone because of their sensitivity to sea level rise, storms and increased human utilization. Previous efforts to protect shorelines have largely involved constructing bulkheads and seawalls which can detrimentally affect nearshore habitats. Recently, efforts have shifted towards “living shoreline” approaches that include biogenic breakwater reefs. Our study experimentally tested the efficacy of breakwater reefs constructed of oyster shell for protecting eroding coastal shorelines and their effect on nearshore fish and shellfish communities. Along two different stretches of eroding shoreline, we created replicated pairs of subtidal breakwater reefs and established unaltered reference areas as controls. At both sites we measured shoreline and bathymetric change and quantified oyster recruitment, fish and mobile macro-invertebrate abundances. Breakwater reef treatments mitigated shoreline retreat by more than 40% at one site, but overall vegetation retreat and erosion rates were high across all treatments and at both sites. Oyster settlement and subsequent survival were observed at both sites, with mean adult densities reaching more than eighty oysters m−2 at one site. We found the corridor between intertidal marsh and oyster reef breakwaters supported higher abundances and different communities of fishes than control plots without oyster reef habitat. Among the fishes and mobile invertebrates that appeared to be strongly enhanced were several economically-important species. Blue crabs (Callinectes sapidus) were the most clearly enhanced (+297%) by the presence of breakwater reefs, while red drum (Sciaenops ocellatus) (+108%), spotted seatrout (Cynoscion nebulosus) (+88%) and flounder (Paralichthys sp.) (+79%) also benefited. Although the vertical relief of the breakwater reefs was reduced over the course of our study and this compromised the shoreline protection capacity, the observed habitat value demonstrates ecological justification for future, more robust shoreline protection projects.

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Temperatures are rising in most temperate and polar environments, and a welldocumented effect of this change is a poleward range shift by a wide variety of terrestrial and aquatic species. In the northern Gulf of Mexico (GOM), an increasing number of tropical species have recently become established among the extant warm-temperate fauna. These include a diversity of tropical fishes, manatees, green turtles, warm-water corals, and black mangroves. The impact of these species may be profound, primarily because temperate species are restricted from shifting northward by the North American land mass. Thus, as tropical species expand northward in the GOM, they must interact with the extant species and potentially compete for essential resources or become prey for each other. Here we focus on tropical immigrants capable of transforming the vast and highly productive seagrass systems of the northern GOM, emphasizing herbivorous parrotfishes and comparing their impact with endemic seagrass-resident fishes. Increased numbers of these herbivores (plus green turtles and manatees) would likely shift detritus-based food webs in seagrass meadows to webs dominated by direct consumption of seagrasses. We provide estimates of some expected consumption rates and effects of these tropically associated seagrass herbivores and predict that the consequences of the increased tropicalization of northern GOM seagrass meadows will be: substantially reduced standing crops and structural complexity of seagrass meadows; increased energy flux through grazing food webs; and a greatly reduced nursery role that will result in much smaller adult populations of those finfish and shellfish species that rely on seagrasses as nurseries.

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Oyster reefs and seagrass beds are being lost worldwide at alarming rates. These habitats provide many services to humankind and, thus, much effort has been dedicated to their restoration. Here, we examine the efficacy of created oyster reefs at enhancing seagrass beds through the amelioration of hydrographic conditions and water quality. We carried out a field experiment in the Northern Gulf of Mexico where we compared areas shoreward of created reefs with adjacent reef-free areas over several years using a before-after control-impact (BACI) design. The reefs were built with oyster shell, measured 65 m, and were placed at circa 100 m from the shoreline to ensure subtidal conditions and enhance oyster recruitment. The BACI results showed few and disparate effects of the reefs, even when distance from the reef was factored in. However, we found a temporal increase in seagrass cover throughout all the experimental area (i.e. including both reef and control plots) following reef deployment. Interestingly, further analysis with satellite imagery showed the experimental area had higher seagrass cover 5 years after reef deployment than it did before reef deployment, but such increase was not observed for nearby areas. In concert, the results suggest "shadow" effects for the reefs examined, where positive effects on seagrass beds extend beyond the area directly shoreward from the reef. Oyster reef restoration may have positive impacts on shallow seagrass beds in turbid, high-energy systems; however, more work on the extent and mechanisms for this interaction is needed.