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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.

October 2008


Trouble in Paradise: Multiple Indicators Used to Assess Water Quality in Kaua`i, Hawai`i
Hypoxia in a Small Estuary: Macroalgae, Rather than Phytoplankton, are Running the Show
Fifty-Five Years of Fish Kills in Texas: An Examination of Causes and Sources
Are Open Bays As Important As Salt Marshes for Brown Shrimp Nurseries in Louisiana?

Trouble in Paradise: Multiple Indicators Used to Assess Water Quality in Kaua`i, Hawai`i

Although the crystal-clear waters of Hawai`i look very different from other U.S. coastal areas, they can suffer from many of the same water quality impairments as temperate waters, including high coliform bacteria counts and nutrient concentrations. A recent study of an embayment on the north shore of the Hawaiian island of Kaua’i revealed that the likely sources of these pollutants are familiar as well.

In response to previously-conducted monitoring which documented occasionally high concentrations of fecal indicator bacteria (FIB, i.e., enterococci and Escherichia coli) and nutrients, these investigators used multiple approaches to track down the sources of these pollutants. Samples taken from nearshore ocean waters, groundwater, river water, and streams were analyzed for nutrient and FIB concentrations. Land use was also assessed in the watershed, and some fecal bacteria samples were tested for the enterococcal surface protein gene (esp), used as a tracer for human fecal sources.

Nutrient concentrations in groundwater were higher than concentrations in the other categories of sites. Based on the pattern of nitrate results and the accompanying land use data, the authors concluded that agriculture is a likely source of nitrogen to the coastal ocean. FIBs were more likely to occur in stream and river samples than in the nearshore or groundwater, suggesting a runoff source of fecal bacteria. Data from the esp gene testing did not indicate human fecal sources for bacteria in stream samples. However, esp genes were present in groundwater and the ocean, suggesting failure or inefficiency of septic systems in the area. The next logical step is to undertake further research and monitoring to track down bacterial sources.

As shown by this study, the simultaneous use of a variety of approaches to assess water quality can provide more complete information than single-parameter monitoring, helping managers to prioritize their efforts. These approaches were successful here despite the hydrological complexity of the system.

Source: Knee, K. L., B. A. Layton, J. H. Street, A. B. Boehm, and A. Paytan. 2008. Sources of nutrients and fecal indicator bacteria to nearshore waters on the north shore of Kaua’i (Hawai’I, USA). Estuaries and Coasts 31(4): 607-622. (View Abstract)

Hypoxia in a Small Estuary: Macroalgae, Rather than Phytoplankton, are Running the Show

While hypoxia is a problem common to many estuaries, the mechanisms driving it differ widely from system to system. In order to predict or mitigate hypoxia, it is critical to understand what is causing it in a given estuary. In one small Chesapeake tributary, Onancock Creek, careful monitoring coupled with a relatively simple diagnostic dissolved oxygen model enabled important features of the creek’s DO dynamics to be captured and better understood. The creek exhibits two distinct hypoxia patterns: diurnal fluctuations typical of shallow systems (hypoxia at night but high, even supersaturated, DO during the day) and prolonged hypoxia following rainfall events. Modeling results offered an explanation based on macroalgal, rather than phytoplankton, control of the system. During fair weather, the creek tends to become anoxic at night when plants are not producing oxygen, but then as macroalgae pump out O2 during the day the creek is re-oxygenated. Because this system is shallow, there is not the stratification that results in bottom water DO depletion in larger systems like the mainstem Chesapeake. As in other systems, rainfall brings high nutrient loads to the creek and also causes increased turbidity and concomitant decreased light penetration. While the nutrient loads stimulate blooms in the creek, the inhibited light levels do not allow the plants to produce enough oxygen to increase daytime DO levels and disperse the hypoxic conditions.

This study highlights the importance of understanding the drivers of hypoxia. Use of a model such as this one can allow managers to predict the timing and extent of hypoxia in shallow systems.

Source: Shen, J., T. Wang, J. Herman, P. Mason, and G. L. Arnold. 2008. Hypoxia in a coastal embayment of the Chesapeake Bay: A model diagnostic study of oxygen dynamics. Estuaries and Coasts 31(4): 652-663. (View Abstract)

Fifty-Five Years of Fish Kills in Texas: An Examination of Causes and Sources

No one likes to see a fish kill – a mass mortality event in which significant numbers of fish carcasses pile up on a beach or litter a waterway. Although they can occur under normal environmental circumstances, fish kills are usually a graphic indication that something has gone awry in a waterway.

As the saying goes, everything is bigger in Texas, which apparently includes fish kills. Texas is a “hotspot” for fish kills: One study found that between 1980 and 1989 an estimated 159 million fish were victims of fish kills in the state. The same study noted that Texas ranked second behind Florida in the number of events resulting in more than one million dead fish. The size of the problem is matched by the size of a database on fish kills maintained by the Texas Parks and Wildlife Department. A recent meta-analysis of that database examined fish kills from 1951 to 2006, characterizing the state’s history of marine fish kills, including geographic patterns and causes.

Of the 383 million fish enumerated in fish kills during that time span, the most frequent victims were Gulf menhaden (72% of fish killed), striped mullet (13%), pinfish (5%), and Atlantic croaker (2%). The largest number of fish kill events and of fish killed occurred during the warmest months, peaking in August. The leading cause of fish kills was low dissolved oxygen stemming from both physical and biological factors. Other important causes included physical damage or trauma to the fish, cold snaps, and biotoxins produced by harmful algal blooms. Galveston and Matagorda Bays were the scenes of the most, and the most severe, fish kills. Texas is probably a hotspot for fish kills because these two bays are typical of Texas coastal water bodies: shallow and prone to low flushing rates and high temperatures, both of which are conducive to fish kills.

The population of Texas is predicted to double by 2050 with no commensurate increase in sewage treatment capacity. The authors of the study warn that this population density increase will likely bring enhanced nutrient loading, more severe algal blooms, and, possibly, even more fish kills. Effective management of this problem will start with a complete understanding of the sources of fish mortality, as described in this paper.

Source: Thronson, A., and A. Quigg. 2008. Fifty-five years of fish kills in coastal Texas. Estuaries and Coasts 31(4): 802-813. (View Abstract)

Are Open Bays As Important As Salt Marshes for Brown Shrimp Nurseries in Louisiana?

If you were asked to identify the most important nursery habitat for juvenile brown shrimp in the Gulf of Mexico, your instant response would probably be “salt marsh.” This is a logical response, given the immense body of research supporting the importance of salt marsh as nursery grounds for a wide range of fish and crustacean species. However, new evidence indicates that open bays in Louisiana’s Barataria and Terrebone Bays may be equally important nurseries for brown shrimp.

The results of seine and trawl surveys indicated that, while similar-sized shrimp density was approximately twice as high in salt marsh sites as open bay sites, the total number of shrimp in the open bays is likely greater than the marsh because the bays cover a much greater area. Isotopic analysis demonstrated that the shrimp found in the bays were, for the most part, bay residents (less than 10% came from marsh habitats) and had not migrated into the bays from nearby marshes. Open bays were estimated to support about 66% of shrimp numbers in Barataria Bay and 63% in Terrebone Bay.

The author of the study suggests that given these results, coastal restoration efforts in Louisiana should focus on measures such as barrier island restoration and conservation measures that protect both marshes and bays, rather than those that specifically target marshes and neglect open bays.

Source: Fry, B.. 2008. Open bays as nurseries for Louisiana brown shrimp. Estuaries and Coasts 31(4): 776-789. (View Abstract)