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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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2016 December

Contents

Investigating Invasive Introductions
Good News So Far in a Danish Estuary
Grand Reopening
A Diversity of Diversities

Investigating Invasive Introductions

What causes nonnative species to spread and thrive? Some clues from Pacific Northwest eelgrass beds

As goods and people move around the world with increasing ease, so do a range of organisms, escalating the threat of introducing nonnative species to new ecosystems. These nonnatives can have devastating effects when and if they take hold in their new home. But what facilitates their spread once they are introduced, and why do some thrive while others do not? A recent study of nonnative species in eelgrass beds in British Columbia examined methods of introduction and spread.

Generalized linear models and correlation analysis were used to link nonnative species richness and abundance to environmental conditions and potential modes of introduction for ten eelgrass sites. Twelve nonnative species were reported at the sites, six of which are known to have negative impacts on eelgrass bed ecosystems. Although epifaunal species did not show a trend, species richness and abundance of nonnative benthic infauna were highly correlated with aquaculture activities. Climate variables (temperature and salinity) helped to explain the trends in species richness, but not abundance. Taken together, these results suggest that many nonnative species in British Columbian eelgrass beds, particularly benthic species, likely arrive as a result of aquaculture activities, and persist when temperature and salinity conditions are appropriate. Given these results, the authors highlight the need to carefully monitor the transfer of aquacultured species, both in and between regions, to prevent further invasions.

Source: Mach, M. E., C. D. Levings, and K. M. A. Chan. 2016. Nonnative species in British Columbia eelgrass beds spread via shellfish aquaculture and stay for the mild climate. Estuaries and Coasts (July 2016). DOI: 10.1007/s12237-016-0124-y.

Good News So Far in a Danish Estuary

Nutrient control programs appear to have set the Roskilde Fjord on the path to better water quality, but there’s still room for improvement

Even after proven management approaches have been implemented, like reducing nutrient inputs to address eutrophication, pathways toward ecological recovery aren’t always clear and unidirectional. Nowhere is this more true than in estuaries, where the ecosystem’s dynamic nature and connection to the ocean can complicate recovery. In Denmark’s Roskilde Fjord, nutrient reduction programs were initiated in the mid-1980s and early 1990s to address eutrophication, leading to significant decreases in nitrogen (58% decrease) and phosphorus (80%) loadings. Did these reductions lead to improved water quality and estuarine function?

Researchers used a long-term data set to examine trends in water quality and net productivity in the decades since the nutrient reductions. The inner and outer portions of the fjord, which are divided by a sill, exhibited different responses. Analysis revealed a 50% reduction in chlorophyll a correlated with a one-meter increase in Secchi depth in the inner estuary, but these parameters did not exhibit significant change in the outer estuary as it is influenced heavily by oceanic waters. In summer, both parts of the estuary experienced a reduction in heterotrophy, which suggests reduced organic matter decomposition, increased productivity, or both. Overall, the system appears to have reached a new, improved stable state in which N limiting levels are approaching levels comparable to the 1960s. However, further improvements can be achieved if seagrass coverage continues to increase, which, the authors say, will lead to further reductions in turbidity and improved habitat for benthic filter-feeders (even further improving water clarity).

At the same time as this system has been recovering thanks to nutrient management measures, water temperatures have increased by 1.5^oC. The authors note that such increases in water temperature could hamper progress by contributing to summertime stratification and benthic hypoxia. Thus, local physical and long-term climate-related conditions may complicate even strong recovery trends in this and other estuarine systems.

Source: Staehr, P. A.., J. Testa, and J. Carstensen. 2016. Decadal changes in water quality and net productivity of a shallow Danish estuary following significant nutrient reductions. Estuaries and Coasts (June 2016). DOI: 10.1007/s12237-016-0117-x.

Estuarine-dependent nekton flock to a remote reopened inlet to a Texas bay

The value of estuarine nursery habitat to the juvenile phases of estuarine-dependent species (those that spend part of their life cycle in estuaries) is well-known. In the Gulf of Mexico, for example, approximately 75% of commercially and recreationally valuable species spend at least one life stage in estuaries where predation risk is low and food availability is high. But what if they can’t get to the estuary because the road is blocked? What happens when the road re-opens? Such was the case in Mesquite Bay, TX, an embayment that was historically connected to the Gulf of Mexico via the Cedar Bayou. The bayou had been closed or mostly closed for thirty years, but was recently dredged to reconnect the bay to the gulf. Researchers examined trends in fish and shellfish densities and community structure at test sites near the inlet opening and at control sites.

Researchers documented many changes at the impacted sites after inlet opening, including an increase in estuarine-dependent fish, post-larval shrimp, and blue crabs. Some of these increases were dramatic, with densities up to 25 times higher after opening than before. Red drum were totally absent from the test sites before the opening, whereas densities after opening were similar to those observed at control sites. Community structure also changed with the opening, becoming more similar to control sites as a result of an increase in estuarine-dependent species. The authors note that, in addition to the obvious benefits of increasing densities of important commercial species, other ecological benefits may accrue; for example, winter mortality in endangered whooping cranes is related to availability of their blue crab prey, so an increase in blue crabs will likely be good news for these overwintering birds. The authors note that reestablishing historical ingress points to estuaries might be particularly beneficial in cases like this one, where there is no other connection nearby.

Source: Hall, Q. A., M. M. Reese Robillard, J. A. Williams, M. J. Ajemian, and G. W. Stunz. 2016. Reopening of a remote tidal inlet increases recruitment of estuarine-dependent nekton. Estuaries and Coasts (May 2016). DOI: 10.1007/s12237-016-0111-3.

A Diversity of Diversities

Examining the differences between functional and taxonomic diversities of macrobenthic invertebrates in Nova Scotia seagrass beds

Measurements of taxonomic diversity are often used to describe ecosystem health. Generally, a greater diversity of organisms is thought to indicate a relatively healthy site, as taxonomic diversity is considered a measure of ecosystem function. But functional characteristics of species (i.e., morphological, behavioral, and life history traits) strongly influence ecosystem properties. In fact, ecosystem function is highly influenced by what species do rather than just the numbers present. Might it be better to evaluate “functional diversity,” which characterizes species roles within communities based on their traits? And are the two types of diversity (i.e., functional and taxonomic diversity) linked? A study of macrobenthic invertebrates in a gradient of seagrass habitats (bare sediment, bed edge, and bed interior) in Nova Scotia, Canada measured and compared both types of diversity and looked for a relationship between the two.

Measuring taxonomic diversity is relatively straightforward: collect samples of the organisms of interest, identify them to the lowest possible taxonomic level, then calculate diversity indices. Measuring functional diversity is a bit more complex. These authors selected biological traits of macrobenthic invertebrates, such as feeding strategy, motility, and size, that directly or indirectly influence processes important for seagrass ecosystem function. The collected organisms were categorized accordingly and the Rao functional diversity index was calculated. They found that the two types of diversity indices did not always show the same patterns across the different seagrass habitats. Taxonomic diversity generally increased across the habitat gradient from bare sediment to bed interior, while functional diversity did not change or showed a weaker pattern. Although there were positive relationships between taxonomic and functional diversity indices, they were not always linear.

The results suggest that taxonomic diversity may not always be a direct surrogate for functional diversity, and by extension, ecosystem functioning and health. This result has important implications for programs that maintain biodiversity under the assumption that ecological function will also be maintained. The relationship between the two diversity types will be useful to predict the implications of species loss, but needs more study, the authors say.  Regardless, functional diversity may turn out to be a very useful tool for coastal managers.

Source: Wong, M. C. and M. Dowd. 2016. Patterns in taxonomic and functional diversity of macrobenthic invertebrates across seagrass habitats: A case study in Atlantic Canada. Estuaries and Coasts (April 2015). DOI: 10.1007/s12237-015-9967-x.