In the southeastern U.S., the upstream reaches of river-dominated estuary systems often include tidal freshwater floodplain forests (tidal forests). Dominated by bald cypress and tupelo gum trees, these forests are inundated by diurnal tides but with very low-salinity water. Sea level rise is expected to present a challenge to these unique ecosystems, as they could be inundated with freshwater more frequently, and saltier water will likely make its way upstream.
Saltwater inundation of these systems could bring about changes in sediment chemistry, which could have important implications for nutrient cycling and eutrophication. The goal of a recent study of tidal forests along the Georgia coast was to determine whether more frequent freshwater flooding or saltwater intrusion would affect inorganic N and P exchange in tidal forest soils. Soils were collected from three Georgia tidal forests that do not currently experience saltwater intrusion and one that does. Soils were flooded with synthetic river water, simulating increased inundation, and N and P removal from the soils was measured. Soils from the three tidal forests that do not experience saltwater intrusion removed significant amounts of inorganic N from the water column when inundated, while soil from the tidal forest that currently experiences saltwater intrusion released N to the water column. Soils from all four rivers sorbed P in similar amounts. In a second experiment, soils from the three healthy tidal forests were flooded with water with salinities of 0, 2 and 5. Under these conditions, N was released from the soils from all three sites to the water column. Phosphorous removal in this experiment increased with increasing salinity.
These results indicate that saltwater intrusion into tidal forests could increase nitrogen concentrations in the water column, contributing to estuarine eutrophication downstream. But longer-term studies of chemical dynamics in this ecosystem type are needed to corroborate and expand upon these findings.
Source: Jun, M., A. E. Altor, and C. B. Craft. 2012. Effects of increased salinity and inundation on inorganic nitrogen exchange and phosphorus sorption by tidal freshwater floodplain forest soils, Georgia (USA). Estuaries and Coasts 35(July 2012). DOI: 10.1007/s12237-012-9499-6.
The Tamar River estuary in northern Tasmania is the site of one of the earliest European settlements in Australia. For more than 200 years, people have been altering the estuarine ecosystem with urban development, water withdrawals for hydroelectric power, and discharge of sewage effluent. Excessive silt deposition has resulted in almost complete loss of the channel at low tide. A major push to restore the estuary is now underway. A recent study argues, however, that the reasons for the siltation and other degradation need to be better understood.
Conventional wisdom states that the sediment deposition in this system is attributable to poor watershed management and high concentrations of flocculated and suspended materials that get transported upstream by asymmetrical tides. But a historical analysis of changes in the estuary found that the sediment deposition problem might be a symptom, rather than a cause, of the overall degradation of the system. The study found that a history of filling the channel, redirecting flow, and filling tidal wetlands for shoreline development has caused a 30% reduction in the tidal prism. Despite frequent dredging, siltation has persisted for more than 100 years and has been exacerbated by diverting freshwater flow for use in a hydropower plant. Watershed sediment loads in this system, the authors conclude, exert a relatively minor role in maintaining estuarine sediment equilibrium.
The authors offer a number of pieces of restoration advice, including increasing the width and depth of the channel to restore riverine and tidal flow, but not via dredging, which is a short-term solution at best. While freshwater diversion in the upper estuary for hydropower is one of the most significant stresses on this system, the authors recognize that it will be virtually impossible to do away with hydropower. They also acknowledge that this mode of power generation has other environmental benefits such as limiting carbon emissions. A “big picture” view of tradeoffs among ecosystem services is needed in this system, and others, in order to implement a sensible restoration plan.
Source: Davis, J. and I. M. Kidd. 2012. Identifying major stressors: the essential precursor to restoring cultural ecosystem services in a degraded estuary. Estuaries and Coasts 35(July 2012). DOI: 10.1007/s12237-012-9498-7.
Water quality monitoring is an essential component of every coastal management program, but it is time-consuming and expensive. What if you could gather some of those data in near-real time from the air, creating a near-instantaneous snapshot of water quality in a given system? A step in that direction is offered by a study of the use of airborne laser-induced fluorescence (LIF) LiDAR to measure dissolved organic matter (DOM) in Canadian estuarine waters. The aerial system was used to quantify DOM in water bodies draining four agricultural areas in Nova Scotia and Prince Edward Island. Results were compared to DOM and dissolved organic carbon (DOC, a relatively constant component of DOM) concentrations measured in simultaneously collected in situ water samples.
The LIF LiDAR system was effective for measuring DOM, particularly after geopositioning improvements were made in the second year of the study that helped align aerial sampling with ground-based sites. However, this system was not as successful at determining DOC, probably because DOC contains varying fractions of material that do not fluoresce and would therefore not be detected using this system. The data demonstrated that DOM values are significantly higher in rivers and estuaries that drain watersheds impacted by agriculture than in systems dominated by other types of land use.
The overall conclusion from this study is that this type of monitoring system, while expensive to set up and deploy, is worth a look when rapid mapping of the relative magnitude of DOM values over large areas is needed.
Source: Rogers, S. R., T. Webster, W. Livingstone, and N. J. O’Driscoll. 2012. Airborne laser-induced fluorescence (LIF) light detection and ranging (LiDAR) for the quantification of dissolved organic matter concentration in natural waters. Estuaries and Coasts 35(July 2012). DOI: 10.1007/s12237-012-9509-8.
The nursery function of estuaries for juvenile marine fishes has long been studied and emphasized in management programs. Young-of-the-year and other juvenile stages find food and shelter in estuaries before moving out into the big, dangerous ocean. But all nurseries are not created equal, a crucial point to remember for managers. The case of white perch in the Chesapeake Bay system is a good example. A recent study of juvenile white perch captured in two Chesapeake tributaries, one freshwater and one brackish, reveals that freshwater might be best for this species, but there is more to be learned.
Body condition and growth were measured for juvenile white perch captured in two tributaries in the Blackwater River drainage. Body condition of juveniles in the freshwater tributary was greater than that of fish collected at the brackish site when water temperatures were above 9.6oC. Growth rates were similar at the two sites. The difference in body condition may have been due to differences in prey availability, or to the fact that in freshwater the fish did not have to use as much energy to maintain homeostasis in the face of fluctuating salinities. The authors speculate that the better body condition measured in the freshwater environment may confer advantages to individuals such as larger size at maturity and reduced vulnerability to predation.
It will be important to maintain a mosaic of habitat types for white perch, as both fresh and brackish habitats are used as nursery areas. Freshwater areas in particular are at risk from saltwater inundation by sea level rise and winter salting of roads in their watersheds. These freshwater areas may be especially important in smaller drainages where migratory animals are relatively more abundant than resident fish, or in systems where brackish water has low rates of production.
Source: Newhard, J. J., J. W. Love, and J. Gill. 2012. Do juvenile white perch Morone americana grow better in freshwater habitats of the Blackwater River drainage (Chesapeake Bay, MD, USA)? Estuaries and Coasts 35(July 2012). DOI: 10.1007/s12237-012-9512-0.