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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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2015 June

Contents

On the Fringe
Epic Epiphytes
Does Ocean Acidification Poison Pens?
When the Tides Don’t Turn

On the Fringe

Small fringing marshes are often squeezed by development and rising seas. How can managers monitor their health?

Fringing marshes, those narrow bands of wetlands that occur along the banks of tidal rivers or bays, are often overlooked when it comes to monitoring marsh health. However, these diminutive marshes perform the same functions – habitat, storm buffer, contaminant filter – as their larger cousins, and they are also highly vulnerable to human impacts. What impacts do shoreline development have on these marshes? What metrics might provide an indication of their health? One study of 18 fringing marsh sites in southern Maine provided some insight.

The investigators quantified shoreline development at each site, and measured a range of biotic factors that could be considered indicators of ecological health, including metrics related to vegetation and vertebrate and invertebrate communities. A range of abiotic factors was measured as well, but none correlated well with the biotic factors. The ecological metrics most associated with degree of development in a 100-meter buffer zone around the marshes were plant Evenness (a diversity index; more development associated with lower Evenness) and differences in the composition of nekton biomass (developed sites had a higher percentage of mummichogs and a lower percentage of invasive green crabs). Developed sites also had lower densities of both dipteran larvae and benthic nematodes in the high marsh. The nematodes may have been responding to past oil spills in the area, however, so the authors suggest that the dipteran larval density is a stronger metric for examining marsh health. Overall, the degree of shoreline development explained 25 to 48 percent of the variance in these metrics.

One important outcome of this work is that it demonstrates that there are substantial differences in biota between marshes adjacent to developed and undeveloped upland areas, and these differences are worth noting and monitoring. While further work would be helpful in refining these metrics, they comprise a good starting list of appropriate components of a fringing marsh monitoring program.

Source: Morgan, P. A., M. Dionne, R. MacKenzie, and J. Miller. 2015. Exploring the effects of shoreline development on fringing salt marshes using nekton, benthic invertebrate, and vegetation metrics. Estuaries and Coasts (February 2015). DOI:  10.1007/s12237-015-9947-1.

Epic Epiphytes

Study in Florida National Wildlife Refuge finds major epiphyte blooms, but natural vs. anthropogenic nutrient sources are elusive

Even ecosystems relatively free from human impact can be subject to the effects of nutrient enrichment and eutrophication. A recent study in the Great White Heron National Wildlife Refuge in Florida sought to obtain a snapshot of nutrient enrichment in the refuge by quantifying nutrients, chlorophyll a, tissue nutrients in two types of vegetation, and epiphyte percent cover on seagrasses in tidal flats. The investigators were interested in determining seasonal patterns in these parameters and discerning natural vs. anthropogenic nutrient sources to the refuge. What they found was somewhat surprising. Contrary to their hypothesis, water column nutrient concentrations were greatest in winter, the driest season. In addition, the highest epiphyte cover on macrophytes and greatest phosphorus content in seagrass tissues was found at sites furthest from human sources, also counter to their expectations. While it was clear from the results that nutrient enrichment is occurring in the refuge, the investigators were not able to determine whether natural or anthropogenic sources were more important, although spatial patterns in macrophyte nutrient ratios suggest sources from the inhabited islands of the Florida Keys and from the Gulf of Mexico. Field sampling also revealed significant epiphyte blooms: Spyridia filamentosa covered seagrass by more than 40% in two consecutive seasons, which coincided with nutrient increases. This study is the first to document S. filamentosa as an epiphyte, capable of long-term blooms.

The authors recommend the use of nutrient enrichment guidelines such as Numeric Nutrient Criteria to inform management decisions in the refuge, as well as continued monitoring and laboratory experiments to further explore the effects of nutrient enrichment on seagrasses and their epiphytes.

Source: Green, L., B. E. Lapointe, and D. E. Gawlik. 2015. Winter nutrient pulse and seagrass epiphyte bloom: evidence of anthropogenic enrichment or natural fluctuations in the Lower Florida Keys? Estuaries and Coasts (February 2015). DOI: s12237-015-9940-8.

Does Ocean Acidification Poison Pens?

Study examines impacts of declining ocean pH levels and warming temperatures on juvenile pen shells in the Mediterranean

Research has shown that ocean acidification – the decline of oceanic pH levels as a result of rising CO₂ induced by global climate change – can depress the ability of shell-forming organisms, from bivalves to echinoderms to bryozoans, to lay down calcium carbonate. In the Mediterranean, one bivalve of concern is the endemic pen shell, listed as endangered by the European Commission and subject to a range of protective measures. Susceptibility of pen shell juveniles to increased levels of CO₂ (and therefore decreased pH) was tested in field trials, and the effects of simultaneously varying temperature and pH were measured in the lab. For the field trials, juveniles were transplanted to sites of varying natural pHs at a hydrothermal vent area subject to naturally occurring high CO₂ concentrations, as well as to a nearby control site; pH levels at the experimental sites mimicked conditions predicted to occur regularly in coastal waters in the coming and future centuries.

The pen shells were apparently fairly resilient to the effects of increased CO₂, at least to levels predicted to occur in the coming one hundred years: metabolism and shell formation were not significantly affected by small depressions in pH (down to about pH 7.7). As pH declined further, however, the bivalves exhibited decreased growth, increased mortality, decreased oxygen consumption, and changes in shell morphology. In the laboratory trials in which the effects of temperature and pH were tested simultaneously, the investigators obtained mixed results. Growth rates were significantly affected by CO₂ but not by temperature. Respiration rates increased with temperature but were unaffected by CO₂. A significant interaction between temperature and CO₂ was observed for mortality rates: mortality increased with temperature for the trials conducted at the control pH, but at the low pH the highest mortality was observed at the intermediate temperature. Overall, the authors conclude that temperature appears to influence physiological parameters like growth and survival, whereas pH has more of an effect on formation of the calcium carbonate shell. These studies underscore the importance of looking at synergism among multiple stressors, as the results are not always as predicted for single stressors acting alone.

Sources: Basso, L., I. E. Hendriks, A. B. Rodríguez-Navarro, M. C. Gambi, and C. M. Duarte. 2015. Extreme pH conditions at a natural CO₂ vent system (Italy) affect growth, and survival of juvenile pen shells (Pinna nobilis). Estuaries and Coasts (January 2015). DOI: 10.1007/s12237-014-9936-9.

Basso, L., I. E. Hendriks, and C. M. Duarte. 2015. Juvenile pen shells (Pinna nobilis) tolerate acidification but are vulnerable to warming. Estuaries and Coasts (February 2015). DOI: 10.1007/212237-015-9948-0.

 When the Tides Don’t Turn

Two studies find impacts of flood gates include degradation of water quality and an increase in eel migration times

Flood gates are commonly used to hold back water to prevent flooding of low-lying coastal areas. Research has shown that these structures can have detrimental environmental consequences, including serving as barriers for migratory species. Two recent studies in areas where flood gates are common, the Lower Fraser River in Canada and the River Stiffkey, UK, found further evidence for the down-sides of flood gate installation.

The Canadian investigators compared water quality of water trapped above a flood gate to that in nearby reference areas in the tidally influenced freshwater reaches of the Lower Fraser River. Water above the flood gates was found to have significantly lower dissolved oxygen that reference areas (2.4 mg/L compared to 8.4 mg/L), considerably lower than the 6 mg/L standard for hypoxia set by the Canadian government. These hypoxic zones sometimes extended 100 m or more upstream of the flood gates. In the River Stiffkey, which empties into the North Sea, researchers implanted endangered European eels with PIT tags to examine their migratory habits at two tide gates. While escapement was high (98.3% of the eels studied passed the gates), eels in reaches with flood gates delayed their migration significantly when compared to  passing an unimpeded control river reach: speed of migration past the two gates was 2.7 and 9.7 times slower, depending on the gate used. A fish passage orifice in one of the gates seemed to help decrease the delay, although the eels were not observed to pass directly through the orifice (the investigators speculated that the faster migration at this site may have been the result of the gates being open on more occasions when eels initially approached them or lower tides and upstream salinity). Migratory delay can be very costly for the fish, in terms of energy expenditure and predation risk.

Both studies highlight the need for evaluating the design of flood gates in order to improve both fish passage and water quality. With the threat of sea level rise, more flood gates may be considered in coastal areas, and these factors should be considered in their construction.

Sources: Gordon, J., M. Arbeider, D. Scott, S. M. Wilson, and J. W. Moore. 2015. When the tides don’t turn: floodgates and hypoxic zones in the Lower Fraser River, British Columbia, Canada. Estuaries and Coasts (January 2015). DOI: 10.1007/s12237-014-9938-7.

Wright, G. V., R. M. Wright, and P. S. Kemp. 2015. Impact of tide gates on the migration of adult European eels, Anguilla anguilla. Estuaries and Coasts (January 2015). DOI: 10.1007/s12237-014-9931-1.