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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.

July 2007


Is Nutrient Loading a Smaller Problem Than We Think?
Improving Detergent’s Role in Tackling a Messy Problem
Australian Study: Weirs Mean Fish Can’t Go with the Flow
Development Paves the Way for Phragmites Invasion in the Chesapeake

Is Nutrient Loading a Smaller Problem Than We Think?

Coastal science gospel states that eutrophication caused by elevated nutrient loadings has triggered major alterations of coastal ecosystem structure and function. A recent journal article turns this conventional wisdom on its head, making the case that the cause of these problems can be found at the top, rather than the bottom, of the food web. The authors assert that rather than nutrient loadings, the more likely culprit is the depletion of top-level consumers in coastal and estuarine ecosystems. Indirect effects of the removal of large consumers are often indistinguishable from effects of nutrient loading, they argue, and they present evidence gathered from more than 100 studies of coral reefs, rocky intertidal areas, and sea grass beds to support the claim.

For example, the authors report that studies evaluating the relative effects of consumers and nutrient supplies on algal biomass have often concluded that consumer (top-down) effects are greater or equal to those of nutrients. One example they cite takes on the classic model of loss of estuarine seagrass. While common understanding holds that nutrient enrichment leads to epiphytic growth on seagrass, killing the plants by blocking sunlight, cascading trophic effects likely have just as much influence. Epiphytic abundance is also controlled by grazers, the absence of which would have the same overgrowth effect as nutrient enrichment.

This paradigm could have major repercussions for management of coastal ecosystems, considering the research and management emphasis of recent decades on nutrient control. Especially if upper trophic levels have been altered, nutrient reduction is unlikely to help restore benthic habitats, note the authors.  

Source: Heck, K. L. Jr., and J. F. Valentine. 2007. The primacy of top-down effects in shallow benthic ecosystems. Estuaries and Coasts 30(3): 371-381. (View Abstract)

Improving Detergent’s Role in Tackling a Messy Problem

Where fecal contamination of coastal waters is a potential problem, the source of the contamination is crucial, since different approaches may be required to address animal as opposed to human sources of fecal contamination. This dirty problem can be addressed by a common cleaning agent: household laundry detergent. The optical brighteners (OBs) in laundry soap that keep those whites so bright are detectable using fluorometry. When detected in surface waters, OBs usually indicate the presence of wastewater, suggesting a human source for fecal bacteria found in the same area. The bad news: elevated total organic carbon (TOC) can interfere with fluorometric OB detection and compromise these analyses.

One recent study may have found a way to address the problem. The use of a 436 nm emission filter when taking fluorometry data reduced TOC interference more than 50%, enabling improved detection of OBs in Georgia coastal waters high in TOC. The filter was not needed to successfully identify human sources of fecal contamination in Puerto Rican waters, which are typically low in TOC.

While the filter enabled detection of human fecal sources at several polluted GA sites, the method does have some limitations. In areas with high TOC (>40 mg/L) the filter did not help: the OB signal was swamped. Also, storm drains may be a special case because fecal bacteria may or may not regrow in them, while OBs degrade over time, meaning that a human source may persist in waters where OBs have long since degraded. Finally, the authors caution that other organic chemicals fluoresce at the same wavelength as OBs, leading to “false positives” for a human source. Experiments are now underway to determine if this problem can be solved by exposing water samples to UV light, which may preferentially decay the OBs and help differentiate between the detergent and other organic compounds.

Source: Hartel, P. G., J. L. McDonald, L. C. Gentit, S. N. J. Hemmings, K. Rodgers, K. A. Smith, C. N. Belcher, R. L. Kuntz, Y. Rivera-Torres, E. Otero, and E. C. Schröder. 2007. Improving fluorometry as a source tracking method to detect human fecal contamination. Estuaries and Coasts 30(3): 551-561. (View Abstract)

Australian Study: Weirs Mean Fish Can’t Go with the Flow

In at least one estuary down under, dams and weirs were found to impact more than freshwater flow: fish community structure and function seemed to be altered such that the community no longer changes in response to the wet-dry cycle.  

The interannual variation in flow in the Ross River in a dry tropical region of Australia is accentuated by the presence of a series of dams and weirs, leading to near-constant marine salinities throughout the estuary and, typically, a lack of freshwater flow to the estuary. Fish communities were sampled along the length of the river in the wet and dry seasons of a very wet and a very dry year. In the dry year only coastal marine species were found, implying that the lack of freshwater flow, compounded by the effects of the weirs, resulted in a fish fauna along the whole estuary typical of that usually seen in the lower reaches of the estuary only. The lack of a gradient of species along the length of the estuary (usually coinciding with the estuary’s salinity gradient) was also considered indicative of a change in character for the whole system. Although the wet year sampled saw substantial flows, enough even to overtop one of the system’s main dams, the fish fauna did not change substantially. While a few freshwater species were found, the authors believe that these individuals washed over the weir from the upstream portion of the estuary and did not actually represent a switch in the fish community.

These results may indicate a fundamental shift in the broader ecology of this system, which may provide a sneak preview of the impact of global climate change on dry tropical estuaries. In this case, the authors offer a solution: diversion of tertiary-treated sewage into the Ross River to restore freshwater flows there, along with fish ladder installation.

Source: Sheaves, M., R. Johnston, B. Molony, and G. Shepard. 2007. The effect of impoundments on the structure and function of fish fauna in a highly regulated dry tropics estuary. Estuaries and Coasts 30(3): 507-517. (View Abstract)

Development Paves the Way for Phragmites Invasion in the Chesapeake

Invasive Phragmites australis (common reed) is already altering the ecology of tidal marshes throughout North America, and is now beginning to invade lower-elevation sites previously thought to be inhospitable to the plant. What conditions allow Phragmites to thrive? Small-scale studies have already established that the expansion of Phragmites stands is facilitated by disturbance and nutrient enrichment often associated with shoreline development. A new large-scale study of Phragmites invasion in the Chesapeake Bay watershed has shown that not only is coastal development associated with greater invasion rates, but there seems to be a threshold level of development that allows for Phragmites invasion.  

The investigators looked at extent of invasion in 90 wetlands nested within 30 subestuaries of the Chesapeake, and correlated those data with digital images of land use practices in the same areas. They also examined nitrogen content of Phragmites foliage sampled in the subestuaries. Results revealed that Phragmites abundance sharply increased in almost every wetland where development adjacent to the marsh’s borders exceeded 15%. Where less than 15% of the border was developed, but natural land cover of the subestuary’s shoreline was low (<~35%), Phragmites was also abundant, suggesting that invasion is linked to regional as well as local land use. N content of the plants was also markedly higher in watersheds with >14-22% shoreline development.  

While these results are correlative, they could have significant conservation implications for the Chesapeake, as they indicate that land use at multiple scales impacts the Bay’s ecology. This study also gives managers something to watch for: this type of research may lead to methods for predicting Phragmites invasion patterns from land use information.

Source: King, R. S., W. V. DeLuca, D. F. Whigham, and P. P. Marra. 2007. Threshold effects of coastal urbanization on Phragmites australis (common reed) abundance and foliar nitrogen in Chesapeake Bay. Estuaries and Coasts 30(3): 469-481. (View Abstract)