CESN Main Page

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.

April 2004


All Estuarine Nutrient Cycles are Not Created Equal: Lessons from a Tropical Estuary
Infection By Viruses May Aid in Brown Tide's Demise
English Sole Study Confirms Concentration Hypothesis
Big Bend Seagrasses Revisited: Region-wide Changes in Species Distributions

All Estuarine Nutrient Cycles are Not Created Equal: Lessons from a Tropical Estuary

For decades coastal managers have understood the basic connections between estuarine nutrient enrichment and low dissolved oxygen, fish kills, and other undesirable consequences. However, because this understanding is based largely on work done in temperate estuaries of the northern hemisphere, it's possible that we may not know as much as we thought about subtropical systems. Recent research in Australia's Brunswick Estuary suggests that tropical estuaries may work quite differently with respect to nutrient cycling, requiring novel management approaches.

Typical of shallow subtropical estuaries, the Brunswick is dominated by short-lived very high freshwater flows in the summer rainy season. A recent study characterized annual patterns in nutrient cycling and algal biomass in the estuary using relatively simple models of water residence time, mixing, and nutrient availability. The study found that at times of high flow residence time in the estuary is short, and nutrients are washed out to the ocean before they can fuel algal blooms. During these periods the watershed is the dominant source of nutrients to the estuary. During periods of low freshwater input to the system, sewage treatment plants dominate the nutrient input, and longer residence times result in complete nutrient removal by algal uptake. At these times, the ocean may become an important source of nutrients, even in this highly enriched system.

This study suggests that managers of these subtropical shallow estuaries need to understand that temperate models of estuarine nutrient cycling may not hold true for their home systems, especially in the rapid response of the system to nutrient input from the watershed. Relatively simple modeling tools, like the ones used in this study, may be useful for managers of these systems.

Source: Ferguson, A., B. Eyre, J. Gay. 2004. Nutrient cycling in the sub-tropical Brunswick Estuary, Australia. Estuaries 27(1): 1-17. (View Abstract)

Infection By Viruses May Aid in Brown Tide's Demise

Like multiple sequels to a bad horror movie, unwelcome "Brown Tide" blooms have continued to plague New York coastal bays since the mid-1980s and New Jersey bays since 1995. The alga, Aureococcus anophagefferens by name, discolors the water to a coffee brown and may cause mortality and reduce growth and feeding rates in important shellfish species, notably the ecologically and commercially valuable bay scallop and hard clam. Shading by brown tide blooms can also have a negative impact on seagrasses. As with all harmful algal blooms, managers would like to know more about what causes the blooms, and just as importantly, what causes them to stop. The usual suspects in algal removal don't work in this case: brown tide cells are avoided by zooplankton that graze on other algal cells, and shellfish won't filter the cells out of the water after concentrations reach densities commonly observed during blooms.

One potential source of mortality investigated in a recent study is infection by virus-like particles (VLPs). Although infection of brown tide cells by VLPs had been documented previously, the role of these pathogens in bloom termination was unknown. These investigators' results suggest an important role of VLPs in bloom termination: the highest percentage of VLP-infected cells was observed at the end of the bloom, the VLPs observed in this particular bloom appeared identical to those observed in previous blooms, and VLPs cultured in the lab from field samples were able to infect healthy non-infected brown tide laboratory cultures. Stay tuned for further work from these investigators on genetic diversity of viruses that infect brown tide cells, and the relative importance of virus-induced mortality compared to other mortality factors.

Source: Gastrich, M.D., J.A. Leigh-Bell, C.J. Gobler, O.R. Anderson, S.W. Wilhelm and M. Bryan. 2004. Viruses as potential regulators of regional brown tide blooms caused by the alga Aureococcus anophagefferens. Estuaries 27(1): 112-119. (View Abstract)

English Sole Study Confirms Concentration Hypothesis

What causes recruitment variability in fish? One recent theory about fish species whose life history includes an estuarine-dependent stage may add a piece to the puzzle. Referred to as the concentration hypothesis, the theory states that estuaries may represent a population "bottleneck" in which density-dependent mortality may occur in years when settlement in the estuary is high, potentially "dampening" recruitment variability to the adult population. A recent study of English sole populations on the Oregon and Washington coasts examined the connection between estuarine nursery habitat and fishery landings for this important commercial species and provided support for the concentration hypothesis in the process.

The investigators were interested in determining whether the level of production represented by commercial landings could be supported by estuarine production alone. They used trawl data to estimate densities of young-of-the-year sole in four Oregon and Washington estuaries over multiple years, and then used those estimates to project total production of sole for all the estuaries on the Oregon and Washington coasts. In comparing these numbers to population estimates made from landings data, the investigators concluded that indeed the offshore fishery can be entirely supported by estuarine production of recruits. Their results also showed that although larval supply of English sole varied considerably from year to year, the estimated populations of juvenile fish observed in the estuaries in August were relatively constant. Because the area of available nursery habitat has also remained fairly constant, this stable contribution of recruits to the adult population supports the concentration hypothesis.

This study provides a good example of a close connection between availability of estuarine nursery habitat and fishery production. Fishery managers may want to emphasize habitat protection and restoration for species whose populations may be limited by the amount of available estuarine nursery habitat. The approach described in this paper could easily be applied to other species and systems to examine the link between estuarine habitat and fishery production.

Source: Rooper, C.N., D.R. Gunderson, and D.A. Armstrong. 2004. Application of the concentration hypothesis to English sole in nursery estuaries and potential contribution to coastal fisheries. Estuaries 27(1): 102-111. (View Abstract)

Big Bend Seagrasses Revisited: Region-wide Changes in Species Distributions

Coastal managers are always hungry for more trend data describing how things have changed in a given system over time. Without this information, it is difficult to assess whether management actions have had the desired effect, and nearly impossible to determine what emerging issues will need attention and resources. A recent survey of seagrass distribution in the Big Bend region of the Florida Gulf Coast provided precisely this kind of information: Researchers revisited sites surveyed between 1974 and 1980 in order to determine whether and how seagrass distributions have changed. The survey found that while the total number of stations at which three of the four most common seagrass species were found was similar between time periods, other important changes had taken place that suggest that nutrient enrichment of coastal rivers may have increased in this region.

Occurrence of two seagrass species declined in the deepest study areas: the deepest observation of turtlegrass was 1 meter shallower than in the previous survey, and star grass appeared to be missing from its deepest 3 meters compared to the previous study. While occurrence of two species, manatee grass and shoal grass, increased between the two study periods, the researches failed to observe seagrass in several locations, often at the mouths of coastal rivers. The fact that some species were missing from the deeper depths of the earlier survey is consistent with the hypothesis that increased nutrient concentrations in rivers discharging to the area may have resulted in increased phytoplankton abundance in coastal areas, reducing light availability to seagrasses. Therefore, seagrasses may not be able to survive at the deeper depths where light no longer penetrates. Supporting this hypothesis is the fact that the study found increases in seagrass species requiring less light and higher nutrient concentrations.

The researchers suggest that the loss of some seagrass species from the deeper edges of beds and changes in species composition of seagrass beds may be a consequence of nutrient loading in this area. This study highlights the need for long-term monitoring data to discriminate between natural variability and human-induced changes in coastal ecosystems.

Source: J. A. Hale, T. K. Frazer, D. A. Tomasko, AND M. O. Hall. 2004. Changes in the distribution of seagrass species along Florida's Central Gulf Coast -- Iverson and Bittaker revisited. Estuaries 27(1): 36-43. (View Abstract)

Other Florida seagrass research is also described in this issue of Estuaries (see references below).

B. E. Lapointe and P. J. Barile. 2004. Comment on "Seagrass die-off in Florida Bay: Long-term trends in abundance and growth of turtle grass, Thalassia testudinum" by J. C. Zieman, J. W. Fourqurean, and T. A. Frankovitch. 1999. Estuaries: 22:460-470.. ESTUARIES 27(1): 157-164. (View Abstract)

J.C. Zieman, J. W. Fourqurean, and T.A. Frankovich. 2004. Reply to B. E. Lapointe and P. J. Barile's comment on our paper "Seagrass die- off in Florida Bay: long term trends in abundance and growth of turtlegrass, Thalassia testudinum.". Estuaries 27(1): 165-172. (No Abstract)