The recent interest in oyster bed restoration often has little to do with increasing harvest of the delicious bivalves, but is instead focused on the ability of oysters to filter phytoplankton and other particles from the water column. This filtration function not only allows for ecologically important benthic-pelagic coupling, it also can improve water quality. Because filtration capacity is the focus of so many restoration projects, it would be helpful to be able to measure this parameter quickly, continuously, and in the field, rather than having to bring field-collected water samples back to the lab, or to rely on laboratory measurements of filtration.
A recent study of natural and restored oyster reefs in South Carolina used an in situ fluorometry technique to gather filtration data, with encouraging results. Chlorophyll a concentrations (a surrogate for phytoplankton) were measured upstream and downstream of the reefs using both the in situ method and laboratory analysis of collected water samples. Both methods gave similar results, which indicated significant phytoplankton removal by the oysters (a mean of 12.9% removal) in all but one case (in which onshore wind likely played a role in resuspending bottom sediments). Individual clearance rates averaged 1.21 h-1 for the small oysters (mean shell height: 36.1 mm) on the intertidal reefs, which is near the low end of previously measured laboratory clearance rates.
Removal rates were about the same for natural and constructed reefs, and were related more to the size of the oysters and density of the bivalve populations on the reefs than to the age of the reefs. The authors suggest that these results indicate that restored shellfish reefs should be able to bring about water quality improvements shortly after construction. The relative impact of a restored reef will depend on the size, density, species composition, location, and flow characteristics of a given reef. The in situ filtration measurement technique used here should be useful in evaluating current and future reef restoration projects.
Source: Grizzle, R. E., J. K. Greene, and L. D. Coen. 2008. Seston removal by natural and constructed intertidal Eastern oyster (Crassostrea virginica) reefs: A comparison with previous laboratory studies, and the value of in situ methods. Estuaries and Coasts 31 (DOI 10.1007/s12237-008-9098-8).
The Brazilian pepper (Schinus terebinthifolius), a native of South America, is causing many of the same problems in its new North American home that other invasive plants do: formation of monocultures that lead to alteration of community structure and displacement of native plants. In order to combat this alien, it is critical to understand how it spreads; previous work indicated that the primary mechanism of seed dispersal is through consumption of fruits by birds and mammals, who then deposit seeds in feces. Florida researchers had noticed, however, that fruits could often be seen floating in local waterways. Could this be a significant mechanism of dispersal as well?
In order for water dispersal to be successful, the fruits have to remain buoyant for a sufficient amount of time to disperse, the seeds have to remain viable in water, and there needs to be a mechanism for depositing floating seeds above the intertidal zone (they do not grow well in the intertidal). All of these factors were assessed experimentally, and the results indicate that water dispersal is a potential vector for expanding the range of this species. Fruits floated longer at higher salinities (4.9 days at 0 ppt, 6.2 days at 15 ppt, 6.9 days at 30 ppt). After seven days, more than 13% of seeds remained viable in water of all salinities. Calculations indicate that fruits can therefore be transported almost 17 km in fresh water and more than 22 km in water of higher salinities at the sites studied. While wind waves were insufficient to push floating fruits above the intertidal zone, boat wakes plus wind waves did move the fruits into sufficiently dry soils for germination to occur.
The results of this study emphasize the importance of management of this plant in river and coastal habitats where the use of multiple dispersal pathways may increase the rate at which new areas are invaded and new infestation centers are established. Multiple dispersal vectors most likely require multiple management approaches.
Source: Donnelly, M. J., and L. J. Walters. 2008. Water and boating activity as dispersal vectors for Schinus terebinthifolius (Brazilian pepper) seeds in freshwater and estuarine habitats. Estuaries and Coasts 31(5): 960-968. (View Abstract)
Not long ago, nutrient loadings to Boston Harbor were among the highest in the world. A major overhaul of sewage treatment in the area, including treatment upgrades and relocation of an outfall offshore in 2000, was designed to address this problem and clean up the harbor. The cleanup plan resulted in a reduction of organic matter load to the harbor of about 90%. One recent study set out to determine if changes to the harbor’s benthic communities followed.
The reduction in carbon loading associated with the outfall relocation led to a shift to more aerobic conditions in the harbor. This shift paved the way for an increase in Ampelisca amphipods, which need some level of organic enrichment but better water quality than the “old” Boston Harbor could offer. The cover of amphipod mats increased from <20% in 1992 to >60% in 1995-1998. As conditions continued to improve, carbon loading fell below the optimum needed to sustain the amphipod mats (the “tipping point” appears to be about 500 g C per m2 per y), and they slowly disappeared, reaching <20% coverage by 2000 and disappearing by 2005. While they dominated the benthos, amphipod tubes facilitated an increase in biodiversity and bioturbation, which increased the depth of oxidized sediments, improving conditions for other benthic invertebrates.
These results highlight the harbor’s good news: wastewater treatment improvements have tipped the balance back to good benthic habitats by favoring organisms that enhance bioturbation. However, some areas of the inner harbor, especially near combined sewer outfalls (CSO), have not yet experienced much improvement. These areas have not had time to “burn off” their stores of organic matter.
Source: Diaz, R. J., D. C. Rhoads, J. A. Blake, R. K. Kropp, and K. E. Keay. 2008. Long-term trends of benthic habitats related to reduction in wastewater discharge to Boston Harbor. Estuaries and Coasts 31 (DOI 10.1007/s12237-008-9094-z).
The storm surge resulting from Hurricane Katrina in 2005 is now legendary: At 9 m, it was the highest storm surge ever recorded in the U.S. Because of that storm surge, Katrina was one of the deadliest and costliest natural disasters in U.S. history. Just a year earlier, Hurricane Ivan, a storm of similar strength (both made landfall as category 3 storms), hit the northeastern Gulf coast not far from where Katrina landed. Although Ivan was a serious storm too, its impacts were far less severe than Katrina’s and its storm surge was much smaller (3 to 5 m). What caused the two storms to be so different? And why was Katrina’s storm surge so much higher than predicted by the commonly-used Saffir-Simpson scale?
Katrina’s record-high storm surge was likely the result of the specific track taken by the storm, says a new study. The coastal geometries where the two storms hit and the bathymetry of the bottom over which they traveled before making landfall were markedly different. Katrina’s path took the storm across the shallow submerged delta of the Mississippi River, causing a higher surge, while Ivan crossed the sloping continental shelf offshore of Mobile Bay before making landfall. To further illustrate this point, the authors of this study constructed a hypothetical scenario in which a storm with all of Katrina’s characteristics – size, intensity, wind fields -- followed the observed track of Hurricane Frederic (1979). This hypothetical storm had a much smaller storm surge, demonstrating that it was not the storm itself, but the track it followed that led to so much damage.
Currently, the Saffir-Simpson scale is used to predict storm surge for a given hurricane strength. In the case of Katrina, the Saffir-Simpson prediction was tragically low. These authors recommend that a new or modified system be developed that should take into account local bathymetry and coastal geography to more accurately predict storm surge.
Source: Chen, Q., L. Wang, and R. Tawes. 2008. Hydrodynamic response of northeastern Gulf of Mexico to hurricanes. Estuaries and Coasts 31 (DOI 10.1007/s12237-008-9089-9).
The nutrient reduction strategies undertaken in the Chesapeake Bay watershed are well known to most coastal managers involved with water quality issues. The Patuxent River Estuary is a convenient microcosm for the larger Chesapeake, and an interesting case study in and of itself. In order to address serious eutrophication and hypoxia problems there, a ban on phosphate-containing detergents and improved phosphorus removal from sewage treatment plant discharges were implemented in the mid-1980s, and sewage treatment upgrades leading to reduced nitrogen loads began in 1991. A recent analysis of a comprehensive 19-year (1985 to 2003) database provides insight into the resulting changes in water quality in that estuary. Point-source loading of N and P to the estuary declined 40-60% following upgrades to sewage treatment plants, and led to parallel declines in nutrients throughout the Patuxent system. Modest declines in phytoplankton chlorophyll-a and bottom water O2 consumption were also recorded for the upper reaches of the estuary. However, chl-a, turbidity, and net ecosystem production increased in the lower estuary, especially during summer. This degradation of water quality in the lower estuary appears to be linked to increasing net inputs of inorganic nitrogen to the estuary from the mainstem Chesapeake (and to above-average river flows during the mid-1990s).
A key driver of the degradation in the lower estuary might be the proliferation of a species of jellyfish (Mnemiopsis leidyi) that preys on zooplankton. This jump in M. leidyi’s abundance has been linked to declines in an important predator, the sea nettle Chrysasora quinquecirrha. M. leidyi now grazes heavily on zooplankton, which in turn could allow for the observed increases in phytoplankton. These analyses demonstrate both the importance of understanding the potential impacts of food web interactions on water quality, and the value of long-term monitoring data for understanding coastal ecosystem responses to nutrient management.
Source: Testa, J. M., W. M. Kemp, W. R. Boynton, and J. D. Hagy III. 2008. Long-term changes in water quality and productivity in the Patuxent River Estuary: 1985 to 2003. Estuaries and Coasts 31 (DOI 10.1007/s12237-008-9095-y).