CESN Main Page

Coastal & Estuarine Science News (CESN)

Coastal and Estuarine Science News (CESN) is an electronic publication providing brief summaries of select articles from the journal Estuaries and Coasts that emphasize management applications of scientific findings. It is a free electronic newsletter delivered to subscribers on a bi-monthly basis.

You can have future issues delivered to your email inbox on a quarterly basis. Sign up today!

July 2022

Table of Contents

Estimating Marsh Vegetation Cover From Space

Developing a national database for UVVR

Marshes are a dynamic mix of vegetated and unvegetated areas that change in response to the advance and retreat of the marsh edge and other factors such as the formation of interior ponds. The unvegetated–vegetated marsh ratio (UVVR) is a recently developed metric that is useful for evaluating the status of a marsh: Increases in UVVR may indicate a system is experiencing open-water conversion due to sea level rise and reduced sediment input. A national inventory of vegetative cover and UVVR can provide critical baseline data for rapidly tracking trends in salt marsh trajectory across estuary and state scales.

A team of USGS researchers estimated the fractional area of vegetation and UVVR within each 30-meter pixel of cloud-free spectral Landsat imagery taken between 2014 and 2018 for the conterminous U.S. The paper describes calibration, validation, and uncertainty of the estimates, and it also provides aggregated estimates of UVVR and vegetation across each coastal state and 29 major estuaries. It also demonstrates the utility of using UVVR to track changes due to management actions. For example, tidal restoration, sediment placement, and levee breaching led to increased fractional vegetation in Delaware’s Prime Hook, Louisiana’s Bayou Sauvage, and Don Edwards San Francisco Bay National Wildlife Refuges, respectively. This approach can also be used to evaluate vegetation loss (e.g. diebacks in the Everglades caused by Hurricane Irma) and to identify unstable, potentially vulnerable areas (e.g. North Carolina’s Mackay Island).

Although there is high uncertainty in some cases, UVVR is a tool that shows great promise. Work is underway to calculate fractional vegetation for the period-of-record of Landsat (back to 1985), as well as to regularly update the 5-year UVVR snapshot presented here. As the product is refined, it will fill a critical gap for tracking the national status of salt marshes through time and space.

Source: Ganju, N.K. et al. 2022. Development and Application of Landsat–Based Wetland Vegetation Cover and UnVegetated–Vegetated Marsh Ratio (UVVR) for the Conterminous United States. Estuaries and Coasts. DOI: 10.1007/s12237-022-01081-x

CSI: Estuaries

The future is bright for environmental DNA

Environmental DNA (eDNA), which targets DNA molecules shed, excreted, or otherwise left behind by living or dead organisms, is an emerging tool for detecting which species are (or were) present in a particular environment. The methods are cost-effective, non-invasive, and especially useful for the detection of rare or otherwise hard-to-detect species. A 2020 symposium focused on the growing body of eDNA studies in estuaries and its potential applications for management.

Environmental DNA assays are highly sensitive and can be used to track organisms at low concentrations. These methods are currently being used in estuaries to track invasive species (e.g. European green crabs in Maine) and screen for wildlife (e.g. manatees in Florida); and assays are being actively developed for many other applications (e.g. Chinook salmon, invasive mussels, and toxic phytoplankton in San Francisco). In addition to single species, eDNA detection can also be used on a mix of species—known as environmental metabarcoding. In China’s Pearl River Estuary, metabarcoding detected 32% more fish species compared with bottom trawling. This approach could also be used to look at community composition before and after habitat restoration.

Because eDNA involves indirect detection, there are important limitations to keep in mind. These methods don’t yet offer information about organism size, age, life stage, or absolute abundance, and there can be spatial and temporal ambiguity since DNA can be easily transported. Additionally, the technique requires the development of DNA libraries for organisms of interest, and assays developed in one system may not work in others. The general consensus was that eDNA is best used to complement established survey methods, such as seines and electrofishing. However, with continued advancements and standardization in methodology, eDNA assays will be an increasingly powerful tool for understanding the “who, what, when, and where” in estuaries.

Source: Nagarajan, R.P. et al. 2022. Environmental DNA Methods for Ecological Monitoring and Biodiversity Assessment in Estuaries. Estuaries and Coasts. DOI: 10.1007/s12237-022-01080-y

Return to Top

Do Land Conservation Practices Improve Water Quality?

Evaluating impacts of land-use change in Chesapeake Bay

Agricultural land use often results in degraded water quality and increased sedimentation rates in estuaries downstream. Although conservation practices can reduce both nutrient and sediment loads, directly assessing the success of these practices remains difficult. Researchers in Maryland wanted to see if an ecosystem comparison approach would be an effective tool for assessing the effects of land-use changes on downstream water quality.

The team compared the downstream impacts of converting intensive grain agriculture to conservation plantings in two adjacent watersheds within tributaries to Chesapeake Bay with similar land-use histories, regional climate, and hydrodynamics. Since the 1990s, approximately 50% of the agricultural land—corn, wheat, and soy farms—within Trippe Creek (“treatment” watershed) have been converted to conservation plantings. In Goldsborough Creek (“reference” watershed), there has been a more modest 20% reduction in agricultural land use. However, the intensity of agriculture in the remaining cropland has increased (with double cropping and fertilizer use), making the net effect difficult to predict.

Between 2010 and 2016, both creeks experienced decreases in total N and P and chlorophyll a, suggesting improvements as a result of reductions (however modest) in agricultural land use. Sedimentation rates, which provide a continuous record of both pre- and post-conservation conditions, remained fairly steady in Trippe Creek. In contrast, sedimentation increased by approximately 50% in Goldsborough Creek, perhaps reflecting increases in development and agricultural intensity.  

The authors conclude that small-scale comparisons can be a potentially valuable tool for detecting the effects of land-use changes that are much slower to be observed at larger scales. However, resolving changes in water quality was still challenging, which makes it difficult to quantify the benefits of retiring agricultural land. The full value of this comparative approach cannot be realized unless more routine water quality monitoring is put in place.

Source: Palinkas, C.M. et al. 2022. Evaluating Impacts of Land‑Use Change on Water Quality and Sedimentation in Downstream Estuarine Waters: a Comparative Approach. Estuaries and Coasts. DOI: 10.1007/s12237-022-01077-7

Return to Top

The Efficacy of Herbicide in Open Water

Does widely-used fluridone help Delta Smelt in the San Francisco Estuary?

Fluridone is a slow-release herbicide that is often used to control invasive submersed aquatic vegetation (SAV). Because of its low toxicity to non-target species, fluridone has been used in the Sacramento­–San Joaquin Delta for the past 20 years to clear waterways, and more recently, to restore open water habitat for critically endangered Delta Smelt (Hypomesus transpacificus). However, the effectiveness of fluridone depends on maintaining target concentrations in the water for enough time­­—which is more easily achieved in slow-moving systems where most field evaluations have been conducted.

A team of California researchers applied and monitored for fluridone multiple times across two pairs of treated and reference sites in the Delta over 18 months. They found that fluridone concentrations in the water of treated sites were generally below the target range required for SAV control, and no lasting reductions in SAV abundance or large changes in community composition were observed. In the sediment, however, fluridone concentrations were hundreds of times higher than those measured in the water and persisted for months. Although high sediment concentration did not affect SAV abundance or community composition in this study, it is not known whether high sediment loads have an impact on non-targeted species. Surprisingly, fluridone was found in the water and sediment samples from the reference sites, suggesting rapid dissipation by tides from treated areas.

This study highlights the research needed to understand water residence time and fluridone exposure in systems with strong tidal influence, as well as the importance of monitoring sediment. In the meantime, the small remaining population of Delta Smelt in the system may be restricted to areas where SAV is excluded by turbidity, salinity, and water depth.

Source: Rasmussen, N. et al. 2022. Efficacy and Fate of Fluridone Applications for Control of Invasive Submersed Aquatic Vegetation in the Estuarine Environment of the Sacramento‑San Joaquin Delta. Estuaries and Coasts. DOI: 10.1007/s12237-022-01079-5

Return to Top