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Coastal & Estuarine Science News (CESN)

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

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2013 April


Changes in Salinity Regime, Changes in Fish Assemblage in a Southern Australia Estuary
Ditch Plugging in New England Salt Marshes: Is the Cure Worse than the Disease?
Climate Change Could Endanger an Already-Endangered Species: Modeling Suggests Problems Ahead for Delta Smelt
Hypoxic Zone Contributes to Structuring Nekton Assemblages in the Gulf of Mexico, Including Bycatch Species

Changes in Salinity Regime, Changes in Fish Assemblage in a Southern Australia Estuary

The blending of saltwater and freshwater is one of the critical factors contributing to estuarine community diversity. What happens when human intervention messes with that mix? The Murray River estuary, the largest estuarine system in temperate Australia, has undergone major changes in recent decades. In 1940, five barrages were constructed, converting most of the system of lakes and lagoons (such that only 11% of the originally estuarine area remains) to freshwater. Recent years have also seen significant drought as well as water withdrawals, further impacting salinity in the system. Using 25 years of catch data from the substantial fisheries in the system, investigators examined whether the combination of reduced freshwater flow and increased fishing pressure changed the fish communities in both the freshwater and estuarine portions of the system. They were particularly interested in changes to populations of four species of large-bodied, late-maturing native fish that are the targets of local fisheries.

Results indicated that there have been significant changes to fish assemblages, including a shift in species composition and a decline in species richness. Species with rapid growth and early maturation (termed “opportunistic strategists”) increasingly dominated catches, while species with slow growth and late maturation (“periodic strategists”) declined. Age structures of these periodic strategists were truncated, with few fish caught at maximum ages.

The authors are particularly concerned with the changes in age structure of the fishery target species, as a shift to smaller sizes and ages can lead to a reduction in the egg production potential of the population. Management of these species, the authors note, should seek to preserve age structures and rebuild them so that these populations can better withstand environmental variability.

Source: Ferguson, G. J., T. M. Ward, Q. Ye, M. C. Geddes, and B. M. Gillanders. 2013. Impacts of drought, flow regime, and fishing on the fish assemblage in southern Australia’s largest temperate estuary. Estuaries and Coasts 36(February 2013). DOI: 10.1007/s12237-012-9582-z.

 Ditch Plugging in New England Salt Marshes: Is the Cure Worse than the Disease?

Salt marshes have been ditched for many decades as a mosquito control measure, but more recent research has encouraged managers to, well, ditch the idea of ditching because of the process’s impacts on habitat quality and integrity. More recently, “ditch-plugging” has been used as a habitat alteration technique for previously-ditched marsh habitats. Ditch-plugging attempts to reverse the draining effect of a ditch by using wetland soil to block the ditch and back water up onto the wetland surface, forming pools.

New England researchers wondered whether ditch-plugging was really better for marsh habitats than ditching, and so they compared marsh hydrology, surface elevation, and soil parameters of natural, ditched, and ditch-plugged areas in three New England marshes. They were particularly interested in whether these marshes were accreting enough to keep up with sea level rise. The results indicated that marshes in created habitats differed from natural habitats, but differences were more distinct for ditch-plugs than for ditches. Importantly, soil strength, carbon storage, and marsh elevation were lowest in ditch-plug habitats, and water levels were highest. In contrast, ditching itself did not appear to result in lower water levels 70 years later. Taken together, these results suggest that ditch-plugging leads to inhibition of marsh self-maintenance processes, probably resulting from poor drainage and retention of stressors (salinity, sulfides) in marsh soils, which in turn leads to decreased organic matter and dieback of plants that assist with marsh accretion. The study also suggests that ditch-plugs can lead to carbon loss and reduced sequestration of greenhouse gases, as well as habitat loss as marsh areas transition from vegetated to open water areas with sea level rise.

Source: Vincent, R. E., D. M. Burdick, and M. Dionne. 2013. Ditching and ditch-plugging in New England salt marshes: effects on hydrology, elevation, and soil characteristics. Estuaries and Coasts 36 (February 2013). DOI: 10.1007/s12237-012-9583-y.

 Climate Change Could Endanger an Already-Endangered Species: Modeling Suggests Problems Ahead for Delta Smelt

Warm, clear water may sound like a perfect vacation to us humans, but these conditions would spell bad news for delta smelt, an endangered fish endemic to the upper San Francisco Estuary in California. A recent modeling study used global climate change models in conjunction with watershed models to project habitat conditions for the delta smelt, examining potential changes in salinity, temperature, and turbidity, and concluded that the persistence of the small fish in much of its current habitat into the next century appears uncertain.

Researchers used scenarios from two global climate change models assuming two trajectories of greenhouse gas emissions developed by the IPCC to project daily air temperatures and freshwater flows in the estuary. The two models bracket the range of recent climate change projections for California, and the two emissions scenarios were chosen to examine outcomes of a “worst-case scenario” of increasing emissions in comparison with a future in which emissions are substantially curbed. The authors note that present-day observed emissions already exceed those of the more extreme scenario, which is now considered by experts to represent a midrange of possible futures.

Model outcomes generally predicted increased salinity upstream, less variable seasonal salinity patterns, and warmer, clearer conditions in the upper estuary, all of which are likely to be detrimental to the delta smelt population. Projected values of a habitat suitability index also suggested problems ahead for the delta smelt. By mid-century, the position of the low salinity zone in the fall and the habitat suitability index converged on values only observed during the worst droughts of the baseline period (1969-2000).

The authors state that their results should be interpreted with caution because of some of the assumptions and necessary omissions of their models. However, the results can be used in developing climate change planning initiatives, particularly when considering approaches for both water management and habitat restoration that are intended to improve conditions for delta smelt.

Source: Brown, L. R., W. A. Bennett, R. W. Wagner, T. Morgan-King, N. Knowles, F. Feyrer, D. H. Schoellhamer, M. T. Stacey, and M. Dettinger. 2013.  Implications for future survival of delta smelt from four climate change scenarios for the Sacramento-San Joaquin Delta, California. Estuaries and Coasts 36(February 2013). DOI: 10.1007/s12237-013-9585-4.

 Hypoxic Zone Contributes to Structuring Nekton Assemblages in the Gulf of Mexico, Including Bycatch Species

Summer months often bring extensive hypoxic conditions to the bottom waters of the northwestern Gulf of Mexico. Although sessile organisms, “stuck” in one place, are obviously at great risk when they are bathed in low-oxygen waters, mobile organisms such as shrimp that can move away from low-DO conditions may still be affected. Researchers used trawl surveys and environmental data collected in two areas that tend to experience hypoxia in the Gulf to evaluate patterns in both the benthic and pelagic assemblages.

Results revealed a relatively diverse benthic assemblage and a low-diversity pelagic assemblage. The benthic assemblage, which responded more strongly to the presence of the hypoxic zone, was primarily distributed across the shelf when hypoxia was absent, inshore and offshore when it was more moderate, and offshore of the hypoxic zone when hypoxia was severe. The authors further noted that the benthic nekton assemblage was similar to the list of bycatch species taken in the brown shrimp industry.

Shifts in distribution to offshore waters during hypoxic years suggest that the benthic assemblage is displaced to regions of the shelf they might otherwise not inhabit, the consequences of which need to be evaluated. There are also potential effects on bycatch. For example, if shrimpers target the edges of hypoxic zones where both shrimp and bycatch species densities are high, then both harvest and bycatch rates may elevated. These dynamics need to be considered when developing management plans.

Source: Craig, J. K. and S. H. Bosman. 2013. Small spatial scale variation in fish assemblage structure in the vicinity of the of the northwestern Gulf of Mexico hypoxic zone. Estuaries and Coasts 36 (February 2013). DOI: 10.1007/s12237-012-9577-9