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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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2012 November


Why Has South Carolina Been Less Crabby Lately?
Burning Questions about Prescribed Fire Management of Marshes
A Simple Index for Assessing Estuarine Trophic State: pH and DO are the Keys
Hydrology Linked to Phytoplankton Biomass in Two NC Estuaries: Rivers both Deliver Nutrients and Flush them Away

Why Has South Carolina Been Less Crabby Lately?

The blue crab, that iconic and tasty decapod of the mid- and south Atlantic coasts, has been harder to find in the past decade. Significant declines in abundance have been reported in the Chesapeake Bay, Texas, Florida, and Georgia. In South Carolina, fishery landings decreased from 7.1 million pounds in 1998 to just 3.2 million pounds in 2010. These declines could be related to the persistent drought-like conditions during the same time frame, which served to increase salinity in many of the salt marshes that provide nursery areas for the species. Could the drought conditions have led to increased infection by parasites such as the dinoflagellate Hematodinium sp., whose prevalence is greatest in high-salinity waters?

Researchers working in South Carolina’s ACE Basin National Estuarine Research Reserve set out to determine temporal and spatial patterns of crab Hematodinium infection rates, and to identify the environmental factors most associated with infection. Infection was indeed generally positively correlated with salinity, as well as with dissolved oxygen and pH, and negatively correlated with temperature. However, there was no difference in survival between infected and uninfected crabs held in the lab for 75 days. Do these results suggest that infection with the parasite is not to blame for the crab decline? Not necessarily, say the authors: an interesting sub-lethal effect of infection was also observed, which could contribute to the problem. An analysis of the crabs’ morphology revealed that infected crabs had shorter lateral spines than uninfected individuals, making their overall carapace width smaller. It’s possible that this difference means that infected crabs are more susceptible to predators. In addition, the authors point out, little is known about how long crabs can harbor the parasite.

Source: Parmenter, K. J., P. A. Vigueira, C. K. Morlok, J. A. Micklewright, K. M. Smith, K. S. Paul, and M. S. Childress. 2012. Seasonal prevalence of Hematodinium sp. infections of blue crabs in three South Carolina (USA) rivers. Estuaries and Coasts 35 (November 2012). DOI: 10.1007/s12237-012-9556-1.

 Burning Questions about Prescribed Fire Management of Marshes

Controlled burns are often used as management techniques in brackish marshes, serving to reduce fuel loads so unintended fires don’t burn out of control and helping to meet wildlife habitat goals. Burning also stimulates plant growth in marshes, an important outcome if marshes are to accrete to keep pace with sea level rise. But the mechanisms by which fire encourages plant biomass production are not well understood. Removal of the existing plant canopy could let in light to spur new growth, or the ash left behind might serve as fertilizer. New research in marshes managed with prescribed burns in Maryland’s Blackwater National Wildlife Refuge suggests that canopy removal is the most important mechanism there, but that responses are somewhat species-specific.

Sites that are subject to annual prescribed burns and sites that were not scheduled to be burned in the year of the study (but had been burned previously) were manipulated to determine the relative importance of canopy removal and ash deposition. At “no-burn” treatment sites the investigators removed the plant canopy, deposited ash on the marsh surface, or did both (controls included neither manipulation). “Annual-burn” treatments included burning with or without canopy replacement. Results at the “no-burn” sites indicate that while there was no significant effect of ash deposition, canopy removal stimulated both above- and belowground biomass production. Aboveground biomass production was stimulated particularly at the sites dominated by sedges (rather than grasses). At the burned sites, replacing the canopy post-burn resulted in a significant decrease in biomass production both above and below ground. Production increased more in response to canopy removal at sites dominated by sedges.

Because of the magnitude of the response to burning in sedge-dominated communities, the authors suggest that controlled burns may particularly benefit these species, and may even lead to sedge dominance in these communities. Managers should attempt to maximize canopy removal during burns to encourage sedge production, keeping in mind that the stimulation of below-ground biomass is an important component of maintaining marsh elevation.

Source: Bickford, W. A., B. A. Needelman, R. R. Weil, and A. H. Baldwin. 2012. Vegetation response to prescribed fire in mid-Atlantic brackish marshes. Estuaries and Coasts 35 (November 2012). DOI: 10.1007/s12237-012-9538-3.

 A Simple Index for Assessing Estuarine Trophic State: pH and DO are the Keys

Trophic status can reveal a lot about the ecological health of an estuarine or coastal system. Although water quality monitoring can be time-consuming and resource-intensive, it is critical for developing and assessing appropriate management strategies. One recent proposal for addressing this dilemma is to use a new index that requires measurement of only two parameters: dissolved oxygen and pH. Fluctuations in these variables provide an indication of system metabolism and therefore trophic status.

Investigators measured various water quality parameters in 90 diverse Irish estuaries for three years, and used the DO and pH observations to calculate an index score on a scale of 1 to 100 for each system. The results of estuarine categorization using the new index were in good agreement with other measures of trophic status, including two existing and more complex indices commonly used in Europe, as well as with simultaneously-measured parameters such as biological oxygen demand and chlorophyll-a. The authors conclude that the index provides an excellent screening mechanism to categorize estuaries by trophic status, thus providing information that can be used in determining management priorities and appropriate allocation of resources.

DO and pH are already frequently measured in standard water quality surveys, and will become even easier to track with the advent of low-maintenance, continuous in-situ monitoring devices. Continuous monitoring would also address the issue of diel fluctuations in DO and pH that could affect the accuracy of the index. The authors caution that use of the index in highly stratified systems could be problematic, requiring the application of a post-facto correction factor based on expert judgment and existing knowledge.

Source: O’Boyle, S., G. McDermott, T. Noklegaard, and R. Wilkes. 2012. A simple index of trophic status in estuaries and coastal bays based on measurements of pH and dissolved oxygen. Estuaries and Coasts 35 (November 2012). DOI: 10.1007/s12237-012-9553-4

Hydrology Linked to Phytoplankton Biomass in Two NC Estuaries: Rivers both Deliver Nutrients and Flush them Away

Knowing that nitrogen loading fuels phytoplankton blooms in estuarine systems doesn’t always help identify a direct link between N loads and chlorophyll-a concentrations. This complexity arises because many other factors, including light availability, residence time, and losses such as grazing all affect chl-a. The effect of freshwater inputs to river-dominated estuaries on chl-a accumulation can be especially difficult to predict since discharge simultaneously controls both N loading and the amount of time phytoplankton spend in the estuary before being flushed out.

A recent study of two river-dominated systems in North Carolina, the Neuse and the New River Estuaries, aimed to quantify these dynamics by carefully monitoring nutrients, phytoplankton, and river discharge over the course of three years. The two systems are geographically close, and thus should be exposed to similar meteorological conditions, but quite different in size and discharge rates. Over the course of the study, both systems experienced a range of conditions, from droughts to floods. In both systems, chl-a concentrations were at maximum levels when flushing times were approximately 10 days. With shorter flushing times (meaning faster discharge rates), chl-a levels were lower, likely because phytoplankton did not reside in the estuary for long enough to take advantage of the pulse of nutrients that accompany increased freshwater flow. With longer flushing times, chl-a was also lower than peak levels, perhaps because less nutrients were delivered to the estuary, but losses such as grazing and sedimentation could also have contributed.

The fact that chlorophyll in these two systems had such similar responses to changes in flushing time is remarkable, given that they have a 10-fold difference in discharge. These results suggest that flushing time is more important for explaining downstream responses than just discharge or N input alone. Teasing out these relationships is important, given that river discharge is likely to be affected by both climate forcing and human activity in the future. .

Source: Peierls, B. L., N. S. Hall, and H. W. Paerl. 2012. Non-monotonic responses of phytoplankton biomass accumulation to hydrologic variability: A comparison of two coastal plain North Carolina estuaries. Estuaries and Coasts 35 (November 2012). DOI: 10.1007/s12237-012-9547-2.