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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.

September 2004


New Lessons from the Hudson River "School"
Hypoxia in the Chesapeake: A Look at History and the Future
Shore Crabs Push Dungeness Out of the Neighborhood in WA Restoration Effort
Sediment Cores Provide Wealth of Information in the Neuse and Pamlico

New Lessons from the Hudson River "School"

A long-term data set from the Hudson River estuary has yielded new information about the structure and stability of the fish community there as well as some important lessons for conducting biota monitoring programs. While it is common to use measures of organism abundance, diversity or community structure to evaluate the effects of environmental change or habitat restoration, detecting patterns can be difficult due to the inherent interannual variability of communities. In addition, as this study demonstrates, controlling factors may take place outside your region or time frame of study.

The investigators analyzed a 21-year (1980-2000) database of beach seine surveys conducted from late August to mid November in the Hudson River estuary to examine relationships between environmental conditions and annual community composition. The analysis revealed that the significant interannual variation in fish abundance observed in the fall was most closely correlated with summer river flows (from three months previous to the surveys themselves). In addition, the community changed significantly over a longer time frame: Atlantic silversides, an annual marine species, increased in abundance, while fresh water species and estuarine residents such as white perch declined in abundance and diversity. Because the observed declines occurred despite recent improvements in water quality in the estuary, these long term changes are not likely due to environmental degradation. The authors hypothesize that invasive zebra mussels, already shown to have an impact on lower trophic levels in the estuary, may be responsible for these changes via their potentially far-reaching impacts on the estuarine food web.

The authors state that estuarine communities may be difficult to study because factors outside the estuary can contribute so significantly to interannual variability in species abundances. However, they stress that this interannual variability must be characterized in order to reveal any longer-term shifts that might be related to environmental change or habitat restoration.

Source: Hurst, T.P., K. A. McKown and D. O. Conover. 2004. Interannual and long-term variation in the nearshore fish community of the mesohaline Hudson River estuary. Estuaries 27(4): 659-669. (View Abstract)

Hypoxia in the Chesapeake: A Look at History and the Future

The Chesapeake Bay, the birthplace of our understanding of the relationships among estuarine nutrient loading, eutrophication and hypoxia, was the site of a new and revealing study of these dynamics using long-term data sets. The authors examined a 52-year record of dissolved oxygen (DO) in the Bay and a similarly lengthy database of nitrate (NO3-) loadings from the Susquehanna River. They also examined the influence of fresh water inflow on NO3- loading and hypoxia.

The study revealed that although summer hypoxia has occurred in the Chesapeake since the 1930s, it has gotten significantly worse since 1950. While 38% of the bottom of the mid-Bay experienced hypoxia in the 1950s, 100% of that same area was affected in 2001. While NO3- concentrations in the Susquehanna peaked in 1989 and then declined, loadings to the Bay remained relatively constant due to an increase in average river flow in the 1990s. Most intriguing was the finding that in the 1990s, even in low river flow/low nitrate loading years (compared to the 1950s), extensive hypoxia occurred. The authors suspect that a change in the Bay's ecosystem has made it more susceptible to hypoxia; such changes could include increases in anoxia-driven phosphorus and ammonia flux out of the sediments which stimulate algal growth, or reductions in removal of phytoplankton due to depleted populations of filter-feeding organisms.

Although the study found no evidence that hypoxia is slowing or ceasing, it did conclude that managers are on the right track with current nutrient reduction goals. Their model demonstrates that reduction of hypoxia to acceptable levels would require a 41% reduction in current nitrate loading rates, which is nearly identical to the 40% reduction proposed by the 1987 Chesapeake Bay Agreement. However, they caution, reaching this ambitious goal will get harder as global climate change could bring about increases in river discharge and therefore even more nutrient loading and hypoxia.

Source: Hagy, J. D., W. R. Boynton, C. W. Keefe and K. V. Wood. 2004. Hypoxia in Chesapeake Bay, 1950-2001: Long-term change in relation to nutrient loading and river flow. Estuaries 27(4): 634-658. (View Abstract)

Shore Crabs Push Dungeness Out of the Neighborhood in WA Restoration Effort

Dungeness crabs, a signature Pacific Northwest delicacy and basis for an important commercial fishery, seem to be delicate in more than just taste: Dungeness were "bullied" out of oyster reef habitat created for them by more aggressive yellow shore crabs in a mitigation project in Gray's Harbor, WA. A recent study of reefs created to mitigate impacts of dredging in Gray's Harbor found that at first the mitigation seemed to be working: Dungeness settlement and recruitment on newly created reefs was high. In subsequent years, newly settled Dungeness were rare while yellow shore crabs were very abundant. Further field and laboratory investigations revealed that settlement of Dungeness crab larvae was lower both in old shell deposits containing shore crabs than in newer deposits (where there were no shore crabs), and in pans of shells containing tethered shore crabs. Although the shore crabs did not prey appreciably on the Dungeness, they were strongly dominant over the Dungeness, exhibiting aggressive behaviors and preventing the Dungeness from seeking shelter in laboratory reefs even in the presence of a predatory fish.

Prior to the mitigation project, there was no reason to suspect that yellow shore crabs would have any significant interactions with Dungeness. The effect became obvious when the shell reef deteriorated more quickly than anticipated due to sedimentation and turbulence, which reduced the three-dimensionality of the reef and possibly increased competition for space between the two species. In more recent years, an apparent shore crab recruitment failure has allowed a resurgence of young Dungeness found at the mitigation sites. The authors remind readers that efficacy of restoration and mitigation efforts may hinge on the population dynamics of species that have not been considered of consequence to the target species.

Source: Visser, E. P., P. S. McDonald and D. A. Armstrong. 2004. The impact of yellow shore crabs, Hemigrapsus oregonensis, on early benthic phase Dungeness crabs, Cancer magister, in intertidal oyster shell mitigation habitat. Estuaries 27(4): 699-715. (View Abstract)

Sediment Cores Provide Wealth of Information in the Neuse and Pamlico

Reconstruction of an estuary's environmental history can be difficult due to data limitations. This critical task was tackled in a recent study of human influence on water quality in North Carolina's Neuse and Pamlico estuaries by making extensive use of the oceanographic version of time capsules buried in the sediments. While it has become relatively common to evaluate individual parameters in sediment cores to examine estuarine environmental histories, these researchers examined an extensive suite of parameters in a set of intact sediment cores to reconstruct a record of human activity in the basins for the past several hundred years. Evidence of human influence on the estuaries was clear: sedimentation rates, nutrient and carbon loading that could be associated with farming, and input of heavy metals from a variety of human activities (cadmium from phosphate mining, for example) all increased over the time horizon of the cores. Multiple indications of relatively recent eutrophication include increased silicate, which indicates an increased incidence of diatom blooms, and a shift in the relative proportion of centric (planktonic) to pennate (benthic and epiphytic) diatoms. The authors speculate that the relative decrease in epiphytic diatoms may be due to the loss of one of their favorite substrates, submerged aquatic vegetation. Diatom species richness and diversity also declined throughout the cores.

These paleoecological techniques have been used in other systems as well, notably in the Chesapeake Bay. The same trends in water quality have been observed there, but the changes found in the Neuse and Pamlico occurred much later than in the Chesapeake system (around 1900-1960, as opposed to around 1760-1800 in Chesapeake Bay).

This approach provides such a wealth of information with a concentrated data collection effort that it would likely be useful in other estuaries and coastal areas where depositional areas can be identified. The resulting data, as shown here, can be used to determine historical water quality, to discriminate between natural and human-induced changes in environmental conditions, and to test effectiveness of management initiatives. One large advantage to this approach is clear here: many of the changes observed in the cores took place before environmental monitoring programs were in place, so they could not have been observed with more traditional techniques.

Source: Cooper, S. R., S. K. McGlothlin, M. Madritch and D. L. Jones. 2004. Paleoecological evidence of human impacts on the Neuse and Pamlico estuaries of North Carolina, USA. Estuaries 27(4): 617-633. (View Abstract)