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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 September


Mississippi River Diversion Does Not Hurt Fisheries, Study Says
Turning up the Heat in a Mediterranean Bay May Lead to More Hypoxia
To Determine the Health of this Estuary, Ask the Phytoplankton and the Benthos
Hypoxia Found to be a Road Block to Zooplankton Migration in the Northern Gulf of Mexico

Mississippi River Diversion Does Not Hurt Fisheries, Study Says

The state of Louisiana is seafood heaven, accounting for about 75% of all fisheries landings in the U.S. Gulf of Mexico. Most of the commercially and recreationally important species there rely on coastal wetlands at some life stage, but those habitats are severely threatened, often by the construction of levees and flood control structures that cut off river flow and sediment inputs. In order to counteract this harmful loss of critical habitat, river diversions have been constructed that reintroduce freshwater and sediments to hydrologically isolated areas. Use of these structures has been controversial because of concerns that they might be having negative impacts on fisheries. Is the cure worse than the disease?

Recent research at the Caernarvon Freshwater Diversion in the Breton Sound Estuary, Louisiana, provides evidence that while the nekton community did change after the diversion became operational, changes seem to be positive ones. Data were collected pre- and post-diversion  and in one area now receiving diversion water (“impacted area”) and another that does  not (“control area”). Results indicate that salinities in the impacted area decreased post-diversion. Indeed, nekton community structure changed significantly in the impacted area: individual fish sizes were smaller post-diversion, suggesting that the impacted area is now acting more as a nursery, providing predation refuge for younger fishes. Fish abundance also increased. Nekton species of ecological or commercial importance either increased in abundance in the impacted area (white shrimp, Atlantic croaker, largemouth bass, menhaden) or did not differ from the control area (brown shrimp, spotted sea trout).

The authors propose that the more pronounced salinity gradient imposed by the diversion likely allows for greater species richness in the system, and also observe that the diversion provides enhanced habitat for young fishes in the form of a greater abundance of submersed aquatic vegetation and flooded wetlands. This study indicates that river diversion can be a useful and appropriate managerial tool with a wide range of benefits to estuarine fisheries.

Source: de Mutsert, K. and J. H. Cowan Jr. 2012. A before-after-control-impact analysis of the effects of a Mississippi River freshwater diversion on estuarine nekton in Louisiana, USA. Estuaries and Coasts 35 (September 2012). DOI: 10.1007/s12237-012-9522-y.

Turning up the Heat in a Mediterranean Bay May Lead to More Hypoxia

A nearly year-long study of oxygen dynamics and temperature in a macroalgal-dominated bay  on the Mediterranean island of Majorca indicates that global warming will increase the likelihood of hypoxia in the bay.

The investigators used continuous dissolved oxygen and temperature measurements to examine hypoxia patterns and estimate gross primary production, net ecosystem production, and community respiration in the bay, which is heavily impacted by nutrient inputs from the surrounding town and boating activities. The system was net heterotrophic during the period of the study, probably subsidized, the authors presume, by organic inputs from groundwater and nearby anthropogenic activity (e.g., residential areas, deficient sewage systems, and boats). Oxygen dynamics and metabolic rates depended strongly on water temperature. One analysis revealed that the probability of finding hypoxia in the bay increased significantly and linearly with temperature. As water temperatures are expected to increase with the effects of global warming (and, in fact, have already increased in this bay), the authors expect that more hypoxia will result in this system, and probably in other similar systems.

Increased temperatures are likely to cause a cascade of interacting effects on oxygen dynamics, not only limiting the amount of oxygen that can be dissolved in the water but also reducing survival times of benthic fauna when exposed to low oxygen and increasing the oxygen thresholds for hypoxia-driven mortality. Thus, as temperatures rise and benthic organisms find themselves needing more oxygen, there is less of it available to them. This “perfect storm” of ecosystem responses could be catastrophic for this bay, and others like it.

Source: Vaquer-Sunyer, R., C. M. Duarte, G. Jordà, and S. Ruiz-Halpern. 2012. Temperature dependence of oxygen dynamics and community metabolism in a shallow Mediterranean macroalgal meadow (Caulerpa prolifera). Estuaries and Coasts 35(September 2012). DOI: 10.1007/s12237-012-9514-y.

To Determine the Health of this Estuary, Ask the Phytoplankton and the Benthos

Indices of ecosystem integrity can help managers compare estuaries and examine changes in a given system attributable to human impacts. Appropriate and useful indices need to be based on easily measured parameters and must be able to distinguish healthy from impaired sites. Ideally, indices should also provide spatially and temporally integrated information about the integrity of the system.

Researchers have developed what seems to be a robust index that fulfills all of these requirements, specific to the Río de la Plata estuary in Argentina. A key advantage of the index is that it relies on metrics derived from both the phytoplankton and benthic communities. To devise the index, the investigators first used water quality measurements to classify 23 estuarine sites along a pollution gradient ranging from heavily impacted to relatively pristine. They then examined more than 36 biological parameters to determine which were correlated with the physiochemical classification of the sites. Each metric was evaluated to determine whether it had a reasonable spread of values and an appropriate signal-to-noise ratio, and whether it allowed  researchers to distinguish between the most and least impacted sites. Four metrics were ultimately chosen that were strongly correlated with level of site impairment: density of cyanobacteria, total density of phytoplankton, percent of benthic diatoms known to be pollution-tolerant, and percent of benthic invertebrates that are tanaids (an order of small shrimp-like crustaceans, sensitive to human impacts).

The so-called Index of Biological Integrity of the Río de la Plata (IBIRP) was able to differentiate between the most and least impacted sites in this system. The authors add that the index is flexible, and can be modified and validated in other similar estuaries.

Source: Gómez, N., M. Licursi, D. E. Bauer, E. S. Ambrosio, and A. Rodrigues Capitulo. 2012. Assessment of biotic integrity of the coastal freshwater tidal zone of a temperate estuary of South America through multiple indicators. Estuaries and Coasts 35 (September 2012). DOI: 10.1007/s12237-012-9528-5.

Hypoxia Found to be a Road Block to Zooplankton Migration in the Northern Gulf of Mexico

For some organisms, especially non-motile benthos, hypoxia can kill. But for others, hypoxia means a change of plans: a shift in location, or in migration or predation habits. These changes can result in indirect impacts on growth, reproduction, or survival, which can be either detrimental or beneficial to a given species overall. Because hypoxia is a serious problem in the Gulf of Mexico, and because fishing there has such critical economic importance in the region and in the nation, it is important to tease apart these effects.

Researchers set out to determine what effects summer hypoxia has had on zooplankton dynamics in the northern Gulf, sampling in a low-hypoxia and a high-hypoxia year. Normally these zooplankton would migrate to deeper waters during the day, but when hypoxic conditions were present in bottom waters they tended to be found at shallower depths, hugging the interface between hypoxic and normoxic waters. When bottom waters were normally oxygenated, the daytime depth of the zooplankton was on average 7 m deeper than when bottom waters were depleted of oxygen. Overall, zooplankton biomass did not differ in the water column between the two years surveyed, but individual zooplankters were smaller in the hypoxic year, either to reduce metabolic demand by reducing surface-to-volume ratio, or because the larger individuals were preferentially consumed by predators.

If the zooplankton are forced by hypoxic conditions to stay in shallower, better-lit waters during the day, they are easier prey for visual-feeding fish, which seems like a good thing for the fish. On the other hand, because the zooplankton seem to be congregating near the hypoxic water interface, perhaps these pelagic fishes are being lured closer to suboptimal habitats. The authors are now developing models to assess the net effect of hypoxia on populations of commercially important fish species in order to evaluate impacts on trophic efficiency, bioenergetics, genetics, and metabolic constraints.

Source: Roman, M. R., J. J. Pierson, D. G. Kimmel, W. C. Boicourt, and X. Zhang. 2012. Impacts of hypoxia on zooplankton spatial distributions in the northern Gulf of Mexico. Estuaries and Coasts 35 (September 2012). DOI: 10.1007/s12237-012-9531-x.