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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 2008


Tidal Restrictions Impact Algae to Mammals in Elkhorn Slough
Before and After Chesapeake Nutrient Reductions: New Patuxent Budget Provides Insights
Urbanization, Nutrients, and Macroalgae: Links in Waquoit Bay
Groundwater Contributes Little Nutrient Load to Neuse, But Could be Locally Important

Tidal Restrictions Impact Algae to Mammals in Elkhorn Slough

A growing body of research demonstrates that tidal restrictions in estuaries almost always lead to altered ecological communities. Managers in many places would now like to restore such systems by removing or altering long-existing structures (e.g., dikes, tide gates, culverts), but predicting the effects of such “alterations to the alterations” is difficult. A comprehensive study in California’s Elkhorn Slough provides some of this much-needed data, examining communities from phytoplankton to marine mammals in a set of sites with a spectrum of tidal restrictions.

This comprehensive study found a pronounced effect of tidal restriction on community diversity: species richness was highest at sites with full tidal exchange and lowest at the most restricted sites, with tidally-muted sites falling in between. Differences in species richness are likely the result of several factors, including water quality, which varied strongly with level of tidal restriction. These authors paid particular attention to those species that are the subject of special conservation efforts, finding that full exchange seems to favor native oysters, commercially valuable flatfish, and migratory shorebirds. On the other hand, minimal exchange was found to support two species of special concern: the threatened tidewater goby and the rare California brackish snail.

The implications of this study are discussed in the context of four types of management goals: maximizing biodiversity, minimizing invasive species, preserving native estuarine residents, and managing important estuarine visitors. The goal of any particular management program – such as these four examples or dozens of others – will determine how information such as that presented in this paper is used. For example, if your goal is maximizing estuary-wide biodiversity, it might be best to retain some areas of very restricted tidal exchange if some species are found only in that type of environment. This study thus serves as a model for evaluating multiple estuarine conservation targets and the trade-offs between them, in order to inform complex coastal management decisions

Source: Ritter, A. F., K. Wasson, S. I. Lonhart, R. K. Preisler, A. Woolfolk, K. A. Griffith, S. Connors, and K. W. Heiman. 2008. Ecological signatures of anthropogenically altered tidal exchange in estuarine ecosystems. Estuaries and Coasts 31(3): 554-571. (View Abstract)

Before and After Chesapeake Nutrient Reductions: New Patuxent Budget Provides Insights

Possibly one of the best USA estuaries for which to construct a quantitative mass-balance nutrient budget is the Patuxent River Estuary, a Chesapeake Bay tributary where there has been an abundance of nutrient research and monitoring attention during the past few decades. A nutrient management plan has been evolving since the mid-1980s, making it a good site for examining impacts of management actions. A reasonably complete nutrient budget for the Patuxent provides a number of insights that could inform nutrient management practices there, in the Chesapeake Bay, and in other estuaries.

Using a combination of previous and recent studies, these investigators parameterized multi-year nitrogen and phosphorous budgets for the Patuxent, and used budget results to draw conclusions about ongoing management initiatives. The resulting budgets for nitrogen and phosphorus are well-balanced, with inputs closely matching exports plus internal losses. The news about the estuary’s management is mixed. While point sources of N and P in the upper estuary have clearly been reduced by improvements in wastewater treatment, there is no evidence that nutrient loads to the larger lower estuary have declined in response to management actions. Variability in rainfall makes a huge difference in this system. In fact, because the years after which sewage treatment plant upgrades took place were wetter than the years prior, diffuse source loading to the estuary actually increased after the new controls were implemented. Diffuse source nutrient input variability, a larger source of nutrients than point sources, is larger than the actual load reduction target (40%) for this system.

This study concluded that N loads need to be reduced by about 50% to restore the estuary’s water quality to the pre-eutrophic conditions of the 1960s. While reductions in point sources have been and will be useful and technically feasible, the authors note that more reductions in diffuse sources are needed, a much tougher problem to solve.

Source: Boynton, W. R., J. D. Hagy, J. C. Cornwell, W. M. Kemp, S. M. Greene, M. S. Owens, J. E. Baker, and R. K. Larsen. 2008. Nutrient budgets and management actions in the Patuxent River Estuary, Maryland. Estuaries and Coasts 31(4): 623-651. (View Abstract)

Urbanization, Nutrients, and Macroalgae: Links in Waquoit Bay

Studies in estuaries throughout the world have demonstrated the links between development and nutrient loading, and between nutrient loading and growth of phytoplankton and macroalgae. Few of these studies have been conducted over multiple years or at an array of sites with a range of nutrient loadings. A recent study in Waquoit Bay, Massachusetts, did both, examining macroalgal biomass and nutrient loading in three sub-estuaries with developed, less developed, and forested watersheds over the course of six years.

As might be expected, the overall biomass of macroalgae increased with increasing nutrient loads, including a six-fold and two-fold increase in the two most abundant bloom-forming species (Cladophora vagabunda and Gracilaria tikvahiae, respectively). But eelgrass fared differently: it was only present in the low nutrient load system, perhaps because it was outshaded or outcompeted in the other two subestuaries by the previously-mentioned dominant species. Seasonal differences among systems were noted as well. While biomass in all systems peaked in May/June, the difference between minimum and maximum biomass was greatest where nutrient loads were highest.

As light and temperature increased seasonally, so did eelgrass, but no such relationship was observed for macroalgae, suggesting that light limitation for these species could work differently than for eelgrass. These macroalgae may be victims of their own success, outshading themselves as the canopy grows to some critical thickness. Because the canopy of one of the species was, in fact, thicker than that critical size in the higher loading sites, the authors speculate that perhaps water turbulence might help the macroalgae to overcome potential light limitation.

Given that Waquoit Bay’s macroalgal biomass is similar to that measured in many other estuaries, similar processes may be occurring elsewhere. As urbanization in the Waquoit watershed progresses, it will be important to monitor this system both for its own sake and to provide a model of these processes for other systems.

Source: Fox, S. E., E. Stieve, I. Valiela, J. Hauxwell, and J. McClelland. 2008. Macrophyte abundance in Waquoit Bay: Effects of land-derived nitrogen loads on seasonal and multi-year biomass patterns. Estuaries and Coasts 31(3): 532-541. (View Abstract)

Groundwater Contributes Little Nutrient Load to Neuse, But Could be Locally Important

Groundwater, often out of sight and out of mind with respect to analyzing estuarine processes, is an important contributor to nutrient loadings in some estuarine systems (such as New York’s Great South Bay). Researchers in the Neuse River Estuary, North Carolina, were aware from previous studies there that discharge of groundwater from aquifers directly surrounding the estuary was not likely to be a very significant contributor of nutrient loadings in this system, and that is what their recent study revealed. In the Neuse, groundwater discharge to the estuary accounts for only about 6% of the annual N and P input from all sources but the ocean. However, the study did find some “hot spots” where high inputs of ammonium and phosphate may explain the occurrence of localized phytoplankton blooms or fish kills. The highest nutrient inputs from groundwater were found close to shore, suggesting that groundwater contributions could be underestimated if they are based on observations from the middle of the river.

One interesting aspect of this study was the use of multiple methods for estimating groundwater flux, including direct measurement using seepage meters, use of radon tracers, construction of a simple water budget, and application of Darcy’s law. There was good agreement among the estimates, lending support to their accuracy.

The methods used in this study are straightforward and could easily be applied in other systems, which could be useful as groundwater nutrient flux estimates are lacking in so many systems.

Source: Spruill, T. B., and J. F. Bratton. 2008. Estimation of groundwater and nutrient fluxes to the Neuse River Estuary, North Carolina. Estuaries and Coasts 31(3): 501-520. (View Abstract)