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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.

July 2003


Study Shows Blue Crabs Thrive in Restored Salt Marsh
Impounded Marsh Communities May Take a Pounding
Hyacinth, Pennywort Shown to be Functionally Different
Study Documents Sources and Loadings of N to 34 Estuaries
Network Analysis Suggests Longer-Term Nitrogen Monitoring Needed in the Neuse

Study Shows Blue Crabs Thrive in Restored Salt Marsh

In recent years, a proliferation of salt marsh restoration projects has sought to reverse some of the damage done to these critical habitats by filling, bulkheading, diking and other human activities. However, few studies have been done of how animal populations respond to restoration efforts. Do marsh-dwelling organisms use marshes once restoration is complete? A recent study of blue crab populations in restored and reference marshes in Delaware Bay found that in the case of blue crabs, the answer appears to be a resounding "yes."

Blue crab populations were sampled in newly restored marshes where dikes had been removed from former salt-hay farms and in reference marshes that had been similarly restored 16 years previously. The investigators found that blue crabs rapidly recolonized restored marshes and were equally or more abundant there than in the reference marshes. Sex ratios were the same at the restored and reference sites, as was the incidence of molting in most months, suggesting that growth is supported equally well at both sets of sites. Crabs often grew faster at the restored sites than in the reference marshes, perhaps due to better protection from predators and an enhanced food supply. Restored locations closer to the mouth of the estuary harbored more adult female crabs than sites further up-estuary.

This study provides evidence that marsh restoration projects are successful in creating habitat for blue crabs in the eastern USA. Restoration initiatives may wish to consider location in the estuary when choosing sites, as sites closer to the mouth of the estuary may serve as refuges for large breeding female crabs, which migrate there to spawn, as well as newly recruited juveniles.

Source: Jivoff, P.R. and K.W. Able. 2003. Evaluating salt marsh restoration in Delaware Bay: The response of blue crabs, Callinectes sapidus, at former salt hay farms. Estuaries 26(3):709-719. (View Abstract)

Impounded Marsh Communities May Take a Pounding

A new study of partially impounded marshes in Delaware suggests that while partial impoundments have been managed to provide habitat for some species, they may adversely impact other species and communities. Partial impoundments, created by restricting or eliminating tidal flow through a marsh using extensive dikes, can be managed by adjusting water levels to provide waterfowl nesting sites and feeding areas for small migratory birds. However, in comparing ponds, creeks, and vegetated banks in a partially impounded marsh and a natural marsh, researchers from Rutgers University found that benthic macrofaunal communities often differed significantly between the two. Community composition was different in all habitats studied, and diversity and abundance also differed in some habitats.

These investigators also had the opportunity to study the impacts of major changes in water level in one partial impoundment by sampling before, during and after an engineered low water event when it was drained for repairs. They observed that after the dike was breached, allowing the impoundment to be flooded diurnally, the benthic community in the shallow, subtidal unvegetated mud was dominated by annelids, anemones and nemerteans. During low water, when 50% of the sediment in the impoundment was exposed and the rest covered by only a few centimeters of water, annelids and insects dominated. After reflooding, the community shifted again, becoming overwhelmingly dominated by an opportunistic species of oligochaete. While the results of the study are not definitive, the dramatic shifts in community structure indicate that draw-downs of water level may have severe impacts on the impoundment ecosystem. As in this case, some of those effects may even impact the food sources of the species intended to benefit from the draw-down.

Source: Stocks, K.I. and J. F. Grassle. 2003. Benthic macrofaunal communities in partially impounded salt marshes in Delaware: Comparisons with natural marshes and responses to sediment exposure. Estuaries 26(3):777-789. (View Abstract)

Hyacinth, Pennywort Shown to be Functionally Different

What happens when an invasive introduced species gains a foothold in an ecosystem that already includes a niche occupied by a similar species? In the Sacramento/San Joaquin Delta, the introduced South American water hyacinth, a floating aquatic plant, is one such species, being similar in many ways to the native pennywort. Both species create a structurally complex canopy that provides habitat for many other species, but a recent study set out to determine if the similarity ends there, or if the two plant species are functionally equivalent.

The investigators compared the invertebrate community inhabiting the native stands of pennywort with that found in areas invaded by hyacinth. They also examined the diet of fish collected in both types of areas to see whether the two types of plants provide similar fish habitat by sheltering the same prey items. They found striking differences between the two: the pennywort, which had a higher leaf density but a lower root density than the hyacinth, supported a higher density of canopy-associated insects. Density of benthic and epibenthic invertebrates was typically higher in the native pennywort than in the hyacinth, and the pennywort supported a greater number of native amphipod species, while a non-indigenous and previously unreported amphipod species was chiefly found in the hyacinth. (However, managers will be interested to know that after a removal program had eliminated hyacinth in one area, and pennywort had persisted there for some time, density of the non-indigenous amphipod declined.) The native amphipod found to be more abundant in the pennywort was a major prey item for fish, while the non-indigenous amphipod found in the hyacinth was not.

This study provides evidence that despite appearances, invasive plants can differ functionally from native species, often supporting a different community or providing different habitat values.

Source: Toft, J.D., C.A. Simenstad, J.R. Cordell, and L.F. Grimaldo. 2003. The effects of introduced water hyacinth on habitat structure, invertebrate assemblages, and fish diets. Estuaries 26(3):746-758. (View Abstract)

Study Documents Sources and Loadings of N to 34 Estuaries

When it comes to managing nitrogen (N) inputs to estuaries and other coastal areas, one size does not fit all. Individual estuaries differ in their major sources of nitrogen, and so management approaches to controlling N loadings should vary as well, a message emphasized by a recent comparative study of nitrogen inputs to 34 estuaries on the Atlantic and Gulf coasts.

The authors examined a wide range of N sources to these estuaries from urban, agricultural, and forested watersheds. A large suite of potential sources was considered in the analysis, including N fertilization of crops and lawns, N fixed by crops, N in food for humans and livestock, and atmospheric deposition of inorganic N. Of the 34 estuaries studied, agricultural activities dominated N input for 20, generally located south of Delaware Bay. Eleven were urban source-dominated, most of them in the northeast. These estuaries were characterized by high loadings from point source discharges such as sewage treatment facilities, septic field leachate, and urban runoff. Atmospheric deposition dominated N inputs in three of the estuaries. Crop fertilization was the most important nitrogen source in 18 estuaries, most of which are in Texas and Florida, reflecting the production of multiple crops in long growing seasons. Livestock feed accounted for high N inputs to estuaries in areas with large populations of cows, hogs and poultry, including some mid-Atlantic and southeastern estuaries, notably the Chesapeake Bay.

N retention by different types of watersheds was also examined: urban-dominated watersheds retained the least N (typified by 21% and 29% retention of total anthropogenic N in the Massachusetts Bay and Narragansett Bay watersheds, respectively) while forested watersheds retained the most (85-95%).

The authors recommend tailoring management regimes to the specific sources of N in a system in order to achieve the best and most cost-effective results. They recommend tertiary sewage treatment as the most effective approach in urban watersheds, and improvements in efficiency of fertilizer and manure use in agricultural systems. Because so few systems are dominated by atmospheric inputs, emissions reductions will likely have a significant impact in only a few of these estuaries.

Source: Castro, M.S., C.T. Driscoll, T.E. Jordan, W.G. Reay, and W.R. Boynton. 2003. Sources of nitrogen to estuaries in the United States. Estuaries 26(3):803-814. (View Abstract)

Network Analysis Suggests Longer-Term Nitrogen Monitoring Needed in the Neuse

A modeling study conducted of the Neuse River Estuary provides evidence that in some estuaries, it may take long-term monitoring to determine whether nitrogen loading reduction programs have worked. Like many other estuaries around the country, managers and policy makers in the Neuse have long been concerned with nutrient loading and eutrophication, leading the state of North Carolina to take steps intended to reduce N loading to the estuary by 30%. Using a system of mathematical modeling called network analysis, scientists used data from 16 consecutive seasons (spring 1985 to winter 1989) to examine N cycling and pathways in the Neuse, exploring the relationship between N loading, N use by phytoplankton, and N export to Pamlico Sound.

They found that short-term control of phytoplankton dynamics is more heavily influenced by environmental conditions, rather than N loadings, in this estuary. Low tidal amplitudes in the system lead to long residence times and high rates of recycling: one N atom entering the estuary could be taken up by phytoplankton as many as 35 times before being exported to the Sound. Therefore, N uptake by phytoplankton in the River is not directly related to N loading rates due to the very high N recycling rates in the Neuse. Although the short-term influence of N control on primary productivity may be difficult to discern, the authors state that long-term monitoring should reveal improvements in water quality.

Source: R.R. Christian and C.R. Thomas. 2003. Network analysis of nitrogen inputs and cycling in the Neuse River Estuary, North Carolina, USA. Estuaries 26(3):815-828. (View Abstract)