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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.

May 2005


Striped Bass Eggs Go with the Flow: Pulsed Events and Transport to the ETM
Better Phytoplankton Identification Through Chemistry?
Role Models for Phytoplankton: Reference Communities Developed for Chesapeake Bay
The Coastal Ocean and CO2: Small Area, Large Flux

Striped Bass Eggs Go with the Flow: Pulsed Events and Transport to the ETM

The estuarine turbidity maximum (ETM), a feature of some estuaries where sediment is trapped and resuspended near the salt front, is a particularly good spot for some fish eggs and larvae to spend time. Eggs are masked from predators under cover of high turbidity, while newly hatched larvae benefit from high concentrations of prey. Salinities can also be favorable for fish eggs and larvae. How do fish ensure that their progeny end up in this optimal spot, particularly since most anadromous fish spawn well above the salt front? A recent study in Chesapeake Bay examines the role of freshwater flow and wind on transport of striped bass eggs to the ETM.

The authors first used a regression model to determine that in fact high freshwater flow does seem to enhance striped bass recruitment: mean freshwater flow rates and the number of pulsed freshwater inputs during the April-May striped bass spawning season explained a significant amount of the variability in abundance of young striped bass in the upper Bay in 1986-2002. A hydrodynamic model was then used to examine the effect of pulsed flow events on striped bass egg transport. The transport of model eggs to the ETM depends on the timing of spawning: eggs spawned after river pulse events may have a better chance of ending up in the ETM than those spawned just before or during an event.

This study suggests that all of the ways in which humans change flow regimes (dams and land use changes for example) may influence the transport of eggs to the ETM, in turn affecting recruitment. The authors also point out that because these episodic events seem to influence recruitment, fisheries models may need to include sufficient resolution in physical forcing factors to be able to account for recruitment variability.

Source: North, E.W., R.R. Hood, S.-Y.Chao and L.P. Sanford. 2005. The influence of episodic events on transport of striped bass eggs to the estuarine turbidity maximum nursery area. Estuaries 28(1): 108-123. (View Abstract)

Better Phytoplankton Identification Through Chemistry?

Many estuarine monitoring and research programs require quantitative characterization of phytoplankton species assemblages, usually carried out by identifying and counting cells under a microscope. Far from ideal, this method is time consuming, somewhat subjective, and requires a good deal of expertise to identify species. A more high-tech method was recently developed that will reduce the incidence of microscope-induced eye strain among aquatic scientists. Taking advantage of the fact that different taxonomic groups of phytoplankton contain different photosynthetic pigments or different ratios of pigments shared in common, samples can be characterized and differentiated using high performance liquid chromatography (HPLC) analysis of their pigments. To assist in this method, the CHEMTAX computer program was introduced, which identifies the components of an unknown sample by matching them to a matrix of reference phytoplankton taxa and their pigment ratios.

CHEMTAX was originally developed for oceanic systems, so it was no surprise that a recent study found that use of the CHEMTAX reference matrix in southeastern U.S. estuaries yielded poor results in identifying the composition of phytoplankton samples when the samples were "ground truthed" using the old-fashioned microscopy technique. The investigators then adjusted the CHEMTAX reference matrix for application in South Carolina estuaries by using pigment ratios measured for local species. As expected, the modified matrix yielded better results, although some taxonomic groups still could not be distinguished.

The CHEMTAX approach is promising for estuarine systems but more work needs to be done on incorporating local pigment ratios in the reference matrix. Also, those microscopes can't be put in storage yet: backup microscope analysis will still be required for some samples. Further work on CHEMTAX modifications for estuaries will be worth it, say the authors, as phytoplankton community structure is such a critical parameter for measuring links between human impacts and estuarine health.

Source: Lewitus, A.J., D.L. White, R.G. Tymowski, M.E. Geesey, S.N. Hymel and P.A. Noble. 2005. Adapting the CHEMTAX method for assessing phytoplankton taxonomic composition in southeastern U.S. estuaries. Estuaries 28(1): 160-172. (View Abstract)

Role Models for Phytoplankton: Reference Communities Developed for Chesapeake Bay

Environmental restoration programs continually struggle with designating appropriate unimpaired reference communities to which restored habitats should aspire. It is often difficult to envision what a restored community should look like, especially if the community in question can only be seen with a microscope. The task of developing a set of phytoplankton reference communities (and describing the water quality conditions that support them) for Chesapeake Bay and its tributaries was recently undertaken by a team of researchers. A continuum of desireable-undesireable water quality conditions was divided into worst, poor, mixed, better and best categories, each associated with a range of nutrient and water clarity conditions. Samples from long-term Bay monitoring programs were then placed into these categories according to the samples' nitrogen, phosphorous, and water clarity (Secchi depth) values. Phytoplankton communities from these samples were described using commonly measured metrics including chlorophyll a and biomass of major taxonomic groups.

Among the differences observed between good and poor habitat conditions: phytoplankton communities associated with the least desirable water quality categories exhibited levels of chlorophyll a typical of bloom conditions, levels not observed in the "best" water quality categories. The reference communities associated with the best water quality conditions had consistently low abundances of bloom-forming, potentially harmful species.

This exercise helps identify appropriate nutrient and chlorophyll targets for restoration, which are in close agreement with the stated targets of restoration programs such as the Chesapeake Bay Program. It is also valuable to note that the reference community associated with the best water quality conditions is not a pipe dream for the Bay - it does currently exist, providing hope that restoration can be achieved by encouraging a community that already exists rather than by replacing one that has been lost.

Source: Buchanan, C., R.V. Lacouture, H.G. Marshall, M. Olson and J.M. Johnson. 2005. Phytoplankton reference communities for Chesapeake Bay and its tidal tributaries. Estuaries 28(1): 138-159. (View Abstract)

The Coastal Ocean and CO2: Small Area, Large Flux

Due to the importance of CO2 in ecosystem carbon budgets and its potential role as a greenhouse gas, lots of scientific energy has been expended in constructing CO2 budgets and examining fluxes between the ocean and the atmosphere. Many studies have asked, where are the CO2 sources and sinks and how does CO2 move within and between systems? In examining the role of the oceans in CO2 budgets, the coastal ocean has usually been neglected. Overlooking the coast is understandable as the continental shelf represents only 7% of the area of the oceans, but a recent Estuaries paper points out that the coast's role is much larger than its area when it comes to CO2.

The study gathered literature estimates of CO2 fluxes in coastal ecosystems and integrated them into a CO2 budget for the ocean/atmosphere that included not only the continental shelf but also estuaries and intertidal areas such as marshes and mangroves. Results indicate that the shelf (excluding the intertidal areas) acts as a CO2 sink, and including it in global CO2 budgets increases the amount of CO2 taken up by the ocean by 24%. However, when estuaries and marshes are added to the picture, the coastal ocean actually switches from a CO2 sink to a source, decreasing the amount of CO2 taken up by the global ocean by 12%. These estimates vary with latitude: at high and low latitudes the coast/intertidal areas act as a net source of CO2 but temperate areas are a net sink. Although these are only preliminary estimates, the magnitude of the effect of including coastal areas in these budgets is clear. It will be important to get a better grasp of the role of these areas, as their CO2 properties are likely to change in response to current and future stresses (such as climate change and further eutrophication).

Of course, many simplifying assumptions are necessary in order to undertake a synthesis such as this. In particular, better estimates are needed of estuarine and intertidal area, and differences between processes in "inner" and "outer" estuaries need to be better understood.

Source: Borges, A.V. 2005. Do we have enough pieces of the jigsaw to integrate CO2 fluxes in the coastal ocean? Estuaries 28(1): 3-27. (View Abstract)