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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.

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2020 July

Table of Contents

Identifying the Susceptibility of New Zealand Estuaries to Eutrophication
San Francisco Case Study Uncovers Warning Signs in the Bay
Eutrophic Cove Improves After Two-Decade Delay
One Hundred Years of Shrimp Farming

Identifying the Susceptibility of New Zealand Estuaries to Eutrophication
Conditions that favor nuisance macroalgal growth put systems at risk

First order predictions of how susceptible an estuary is to opportunistic macroalgae and phytoplankton blooms can be made based on the system’s nutrient load and physical properties like volume, inflow, and tidal range. Because these properties don’t require extensive field observations, an assessment can be done relatively quickly and easily.

Researchers in New Zealand developed a model that predicts the susceptibility of estuaries to eutrophication based on the response of primary producers to nutrient concentrations and flushing times. The team then used their model to calculate the eutrophication susceptibilities of 399 estuaries in New Zealand. They identified 108 estuaries with high or very high eutrophication susceptibilities. The most at-risk systems were those where the intertidal area comprises more than 40% of the total, providing suitable habitat for macroalgae to grow.

They also observed trends in the susceptibility of different types of estuaries. The least susceptible were freshwater river mouths and associated lagoons, due to short flushing times that prevent phytoplankton accumulation and low salinities that inhibit macroalgal growth. No fjords and only a small number of coastal embayments were highly susceptible because of their high dilution rates. Overall, the most susceptible estuaries were coastal and tidal lagoons, followed by deep drowned valleys, beach streams, and tidal river mouths. The few areas in New Zealand with consistently low susceptibilities were either undeveloped or they have estuaries that can tolerate greater increases in nutrient loads because of their short flushing times, small intertidal areas, or minimal tidal influx.

Their model—available as a free web-based tool—can be a useful screening tool for managers to determine if more in-depth estuary assessments are needed and to predict potential responses to changing nutrient loads.

Source: Plew, D.R. et al. 2020. Assessing the Eutrophic Susceptibility of New Zealand Estuaries. Estuaries and Coasts. DOI: 10.1007/s12237-020-00729-w

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San Francisco Case Study Uncovers Warning Signs in the Bay

Nutrient pollution risks can be minimized using scientifically grounded strategies

To prevent coastal waters from being degraded by nutrient enrichment, policymakers need the answers to two essential questions: Is the system at risk from high nutrient inputs, and how can that information be used to develop protective policies? A team of USGS researchers used San Francisco Bay as a case study for evaluating the potential risks from nutrient pollution and how they might be addressed.

San Francisco Bay has nutrient concentrations that are higher than those in other eutrophic systems (such as Chesapeake Bay), yet chlorophyll concentrations are comparatively lower in San Francisco and the Bay has not shown classic signs of eutrophication such as hypoxia and recurrent harmful algal blooms. This is thought to be in part because high concentrations of suspended particulate matter result in light limitation of phytoplankton production. To get a complete picture of the current status of the Bay, the team compared extensive water quality data collected over the past four and a half years against historical observations.

San Francisco Bay’s longstanding resilience to the harmful effects of nutrient enrichment is eroding. Nutrient inputs, which come primarily from wastewater, are at levels that can cause degradation. And although chlorophyll concentrations remain relatively low, the researchers found several causes for concern: the presence of four toxin-producing phytoplankton groups, an increase in primary production since 2000, and projected future climate conditions that would increase the magnitude and frequency of algal blooms.

Based on these and other findings, scientists and stakeholders are collaborating on several follow-up activities, including evaluating whether nutrient reductions can reduce algal toxins, establishing management goals as numeric targets (such as chlorophyll thresholds), and determining the nutrient load reduction required to meet those targets—all of which will be used to help create protective policies for San Francisco Bay.

Source: Cloern, J.E. et al. 2020. Nutrient Status of San Francisco Bay and Its Management Implications. Estuaries and Coasts. DOI: 10.1007/s12237-020-00737-w

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Eutrophic Cove Improves After Two-Decade Delay

Tidal freshwater systems can bounce back when phosphorous is reduced, but don’t expect a full recovery

Tidal freshwater systems downstream from cities are especially vulnerable to excess phosphorus, which may cause phytoplankton blooms and reduce water clarity. Although Virginia’s Gunston Cove saw a dramatic increase in phosphorus from domestic wastewater as the population of the Washington D.C. metro area burgeoned beginning in the 1940s, government regulations greatly curtailed phosphorus loading by 95% in the 1980s.

Equipped with a 34-year dataset that began in 1983, researchers were able to measure long-term responses to the reduced input of phosphorus in terms of nutrient concentrations, light transparency, phytoplankton biomass, and vegetation coverage in this shallow embayment of the tidal freshwater Potomac River, where lush, diverse growths of submersed aquatic vegetation (SAV) once thrived.

The analysis revealed a strong but delayed and incomplete recovery. Phosphorus concentrations in the water column decreased slowly and gradually, but a distinct decrease in chlorophyll didn’t appear until 2000, and it took 20 years before a major increase in water clarity was realized and SAV began to return. Not only did the recovery response lag by several years, but the degree of recovery was also less than expected: Although SAV coverage increased dramatically in 2005 and remained strong, it hasn’t increased substantially since, nor has the cove reached its pre-degradation water clarity due to suspended solids.

Although shallow freshwater ecosystems can recover from eutrophication with time, the response will likely be nonlinear with substantial delays. It’s unrealistic to expect a complete return to pre-disturbance levels, especially in areas that have been urbanized since the original degradation.

Source: Jones, R.C. 2020. Recovery of a Tidal Freshwater Embayment from Eutrophication: a Multidecadal Study. Estuaries and Coasts. DOI: 10.1007/s12237-020-00730-3

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One Hundred Years of Shrimp Farming

Aquaculture can alter the landscape of coastal lagoons

Shrimp farming and other marine aquaculture activities occupy large clusters of ponds along the coasts of several continents. Because of evaporation and seepage, pond water must be regularly renewed by the entry of new water, and exchange operations are sometimes needed to prevent the deterioration of water quality. Although it is known that pond management can influence the surrounding ecosystems, the long-term effects of these water management strategies haven’t been thoroughly examined.

A team of researchers used a model to simulate the effects of pond water management in two different cases: One is a hypothetical, highly simplified, sandy-bottom tidal basin surrounded by excavated ponds, and the other is based on Brazil’s Guaraíras lagoon system, which has been utilized for shrimp farming since 1924. In their scenarios, the team altered the rates of water intake and exchange over 100 years, and then they evaluated the resulting morphological alterations, including the formation of new intertidal areas and tidal channels and changes in bed levels and sediment volumes.

The results suggest that shrimp farming distorts flow patterns and sedimentary processes in coastal lagoons. Tidal channels are constantly being formed and deepened, and flood and ebb velocities decrease as a consequence of inlet erosion and increased cross-sectional area. Not surprisingly, the larger the pond, the larger the effects. Shrimp farming also affects the sediment budget of tidal basins: Water intake increases the import of sediment from neighboring beaches, whereas water exchange reduces sediment importation.

This modeling approach could be extended to various comparable systems around the world to provide environmental impact prognostics for coastal planners and enhance long-term aquaculture sustainability.

Source: Roversi, F. et al. 2020. Numerical Modeling Evaluation of the Impacts of Shrimp Farming Operations on Long-term Coastal Lagoon Morphodynamics. Estuaries and Coasts. DOI: 10.1007/s12237-020-00743-y

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