Estuaries have historically been ideal locations for cities, as they provide the natural infrastructure and resources to support commerce and agriculture. But controlling and shaping estuaries to our liking by armoring shorelines with rip rap and bulkheads may come with consequences for intertidal habitats and communities. A study comparing armored and unarmored shorelines in the Duwamish River estuary in Seattle found evidence that, by many measures of habitat suitability, armored shoreline areas are deficient compared to their natural counterparts.
The study examined paired armored/unarmored sites in the Duwamish estuary, more than two-thirds of which is armored, in the industrial heart of Seattle. Habitat and community characteristics differed significantly between the armored and unarmored sites: armored sites had steeper slopes, less riparian vegetation, and warmer substrate temperatures than unarmored sites. Armored sites also had an order of magnitude fewer benthic invertebrates and about 50% lower taxon richness. Gut contents of juvenile chum salmon also differed between the armored and unarmored sites, with more benthic than planktonic or terrestrial prey found in the stomachs of fish caught at unarmored sites. However, not all of the variables differed between the sites: water column temperatures and abundances of water column invertebrates were similar, and gut contents of Chinook salmon at both types of sites preyed mostly on terrestrial insects.
The authors emphasize the need for further studies of this type, as shoreline armoring occurs in urban areas worldwide, and continued human population growth probably means this issue is not going to disappear. Perhaps the design of future urban shorelines could be improved with more understanding of the ecological consequences of particular types of armoring.
Source: Morley, S. A., J. D. Toft, and K. M. Hanson. 2012. Ecological effects of shoreline armoring on intertidal habitats of a Puget Sound urban estuary. Estuaries and Coasts 35 (February 2012). DOI: 10.1007/s12237-012-9481-3.
The common description of an estuary as a meeting place for saltwater from the ocean and freshwater from a river isn’t always a perfect fit. For example, one interesting and under-studied estuarine type is the temporarily open/closed estuary (TOCE), where an inlet to the ocean is closed at some times and breached, either naturally or artificially, at others. The dynamics of these systems depend largely on the state of the inlet. Intermittent breaching of the berm at the estuary mouth may lead to dramatic and swift changes in the system’s physio-chemical environment, which can trigger major biological changes as well.
A study of the Camacho lagoon, a TOCE in southern Brazil, revealed that an artificial breach led to major and immediate changes in estuarine communities. As expected, when the barrier was bulldozed and the ocean flooded the lagoon, salinity increased significantly and became more variable (salinity range was 0.2-1.6 pre-breach, and 1.68-20.38 afterwards). Total organic matter and microphytobenthos both decreased significantly. The shock to the system of the rapid change in salinity and organic matter led to a population crash of macrobenthic invertebrates: biomass was reduced by 50% and density was reduced by 90%. Over time, biomass and density recovered, but the new community structure was significantly different than pre-breach. Statistical analyses demonstrated that the macrobenthic community in the lagoon is primarily structured by salinity and microphytobenthic biomass, which are regulated by the state of the inlet.
Studies of this kind argue for a “big picture” approach to TOCE management. While artificial breaches of this type of system are often carried out in order to control flooding or pollution, ecological impacts of the kind observed in Camacho lagoon should be taken into consideration when evaluating whether to undertake such a project.
Source: Netto, S. A., A. M. Domingos, and M. N. Kurtz. 2012. Effects of artificial breaching of a temporarily open/closed estuary on benthic macroinvertebrates (Camacho lagoon, southern Brazil). Estuaries and Coasts 35 (March 2012). DOI: 10.1007/s12237-012-9488-9.
Eutrophic systems are often characterized by complex feedback loops and overlapping causes and effects, which can result in tangled food webs. In many systems, one result of eutrophication has been a marked increase in blooms of various types of jellyfish and comb jellies, both of which tolerate low-oxygen conditions better than many organisms. A recent description of the eutrophication history of a Danish fjord documents the rise of both of these gelatinous organisms in the system, and the causes and effects of these blooms.
Limfjorden is a 1,500 km2 sound-like system that connects Kattegat with the North Sea. The history of nutrient enrichment in the system dates back to the mid-1900s, which saw an increase in agriculture and pig farming in the watershed. After World War II, fertilizer use increased markedly, and agricultural development accelerated even further after about 1960. Nutrient loading has been associated with algal blooms and subsequent oxygen depletion in stratified bottom waters in the summertime (when settled and decaying algae on the bottom use up the oxygen). After 1960, these consequences became so severe that benthic fauna and fish populations declined precipitously. Populations of benthic filter-feeders such as blue mussels were decimated by the release of hydrogen sulfide from anoxic sediments, which in turn reduced grazing pressure on phytoplankton, allowing for even more algal biomass to thrive and then die off.
Along with all of these effects came the moon jellyfish (Aurelia aurita) and the invasive ctenophore Mnemiposis leidyi (which was observed in Limfjorden for the first time in 2007). Tolerant of hypoxia, both of these organisms are voracious zooplankton consumers, creating another feedback loop that results in algal blooms by relieving grazing pressure on phytoplankton. Gelatinous predation pressure on the algal-grazing zooplankton organisms can be so high that copepods and other mesozooplankton virtually disappear from the system in some years (2008 and 2009 are good examples).
While programs have been put in place to reduce nutrient inputs in this system, these authors contend that far stricter reductions are needed, particularly for nitrogen, in order to force major algal bloom reductions and turn the system around. Further, comprehensive long-term monitoring of the system, including of the dynamics of gelatinous organisms, is needed, along with research focused on their role as both a cause and a result of eutrophication.
Source: Riisgård, H. U., P Anderson, and E. Hoffman. 2012. From fish to jellyfish in the eutrophicated Limfjorden (Denmark). Estuaries and Coasts 35 (February 2012). DOI: 10.1007/s12237-012-9480-4.
Like so many other estuaries around the world, those in the Basque region of Spain are at risk of eutrophication from anthropogenic nutrient inputs in their watersheds. But how much risk? And how can that risk best be evaluated? A recent assessment of eutrophication risk in 14 Basque water bodies utilized two assessment types and compared the outcomes. One of the methods was an adaptation of an approach developed under the Water Framework Directive, the defining legislation protecting all European Union waters. Referred to as WFD-BC, this approach assessed the “risk of failing to achieve good ecological status, due to anthropogenic nutrient pressure” and included different biological, physico-chemical and hydro-morphological quality elements in the evaluation.. Results from this assessment were compared to results using ASSETS, a method developed in the U.S. to evaluate trophic status of a water body based on both primary (chlorophyll a, macroalgae) and secondary (low DO, presence of harmful algal blooms) indicators of eutrophication, using a pressure-state-response framework.
The results indicate that there is no “one size fits all” method to use for evaluating eutrophication risk. Although the two methods generally yielded similar results for the water bodies that were most at risk, there were a number of key differences related to the way susceptibility is calculated by each approach. Sites that scored better with WFD-BC (designated as being at low risk of eutrophication) were given more negative scores (i.e. moderate trophic status) using ASSETS, which may reflect the way dilution and flushing are calculated using the ASSETS approach. ASSETS assumes that estuaries with large volumes and freshwater influences have a greater capacity to dilute and flush nutrients, and thus classify the small-volume Basque estuaries as highly susceptible to enrichment. The authors suggest that the thresholds used for calculating dilution and flushing potentials should be reconsidered in small systems like these.
This type of study comparing two alternate methods of assessment points out the strengths and weaknesses of both approaches. The WFD-BC method exhibited high potential for assessing eutrophication in these systems, but some improvements were recommended, including decreasing the weight given to the benthic and macroalgae components of the assessment, which may skew results.
Source: Garmendia, M., S. Bricker, M. Revilla, Á. Borja, J. Franco, J. Bald, and V. Valencia. 2012. Eutrophication assessment in Basque estuaries: comparing a North American and a European Method. Estuaries and Coasts 35 (March 2012). DOI: 10.1007/s12237-012-9489-8.