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2019 November

Table of Contents

Does Hypoxia Change the Menu?
Putting Small Restoration Projects to the Test
Cues from Chlorophyll
Restored vs. Natural Marsh: Is It All the Same to Nekton?

Gulf of Mexico fish show variable effect of low oxygen events on food web

Every summer, the coastal waters where the Mississippi River meets the Gulf of Mexico experience one of the largest hypoxic events in the world, caused by high nutrient loading from agriculture in this watershed. The effects of hypoxia are known to be complex, and directly influence growth, reproduction, recruitment, susceptibility to stress, and mortality of marine organisms. A new study attempted to characterize the effect of these seasonal hypoxic events on the food web of this region, with particular consideration of the high value of commercial and recreational fisheries in the area.

The authors counted fish caught in research trawls that sampled the bottom and mid-water during the late summer and early fall of 2006, 2007, and 2008. They also analyzed the diets of the and found that some species of fish changed their diets when they were in areas experiencing hypoxia. This change was notable in fish that primarily feed on benthic food or zooplankton, but not in piscivores (fish that eat fish), whose prey was perhaps as equally able to avoid hypoxic areas as the larger predators themselves. The authors also found lower catch rates of many species in areas with hypoxia.

Although more research is needed to scale up and predict the overall effects of hypoxia on this important ecosystem, these findings indicate that changes in fish diets might be expected before overall community changes occur during hypoxic events. Additionally, the research shows that hypoxia will influence individual fish species differently. A more thorough understanding of how declining oxygen levels can affect these dynamics could help predict both broader ecological and commercial effects, and inform ecosystem models.

Source: Glaspie, C.N. et al. 2019. Fish diet shifts associated with the northern Gulf of Mexico hypoxic zone. DOI: 10.1007/s12237-019-00626-x

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Putting Small Restoration Projects to the Test
New framework evaluates long-term effectiveness on water quality in Tampa Bay

Florida’s Tampa Bay is a success story. Reduced nitrogen loading and chlorophyll-a concentrations, improved water clarity, and ultimately increased seagrass cover have all been achieved through various coastal management efforts -- despite 60 percent of the surrounding area being developed. A recent paper developed a decision support tool that matched water quality stations with individual restoration projects in order to assess whether they had a measurable effect on water quality at different spatial and temporal scales.

The framework used records from 45 long-term monitoring stations in Tampa Bay to compare pre- and post results from almost 900 restoration projects categorized into five types (point or non-point source controls; habitat enhancement, establishment, or protection). The analysis showed that decreases in chlorophyll-a were strongly associated with water infrastructure projects for nonpoint sources, point sources, and habitat protection. Decreases in chlorophyll-a were not as well-linked with habitat establishment or enhancement projects, although some associations were found for establishment projects, including mangrove or seagrass planting and the creation of oyster reefs.

The analysis also evaluated the time-frame and number of projects, in combination with one another, that might be expected to produce an improvement in water quality. However, the authors are careful to point out that this logic is not intended to be causal, as in “chlorophyll-a would be reduced by 2 μg/L after 10 years, if five projects are implemented in the future.” Rather, this framework can serve as a tool for managers to set expectations for various restoration actions and their typical aggregate effects, as well as a method of assessing the effect of habitat restoration projects on water quality over the long term.

Source: Beck, M.W. et al. 2019. Assessment of the Cumulative Effects of Restoration Activities on Water Quality in Tampa Bay, Florida. Estuaries and Coasts. DOI: 10.1007/s12237-019-00619-w

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Cues from Chlorophyll
Finding patterns in eutrophication indicators in Biscayne Bay

Globally, eutrophication appears to be increasing in coastal estuaries and bays surrounded by human development. A study based in Biscayne Bay, adjacent to the city of Miami, analyzed 20 years of water quality data from individual stations within the Bay. This research sought to establish whether patterns in chlorophyll a and nutrient concentrations in this population-dense watershed could be used to identify areas in need of management.

The analysis found an overall increase in chlorophyll a and phosphate concentrations over time, which the authors point to an indication of overall eutrophication. The worst water quality was found in the northernmost part of the Bay, which is surrounded by urban development and sees the least tidal flushing. Additionally, stations at the mouth of two canals entering central Biscayne Bay showed large degradation and a history of seagrass die-offs. 

The authors concluded that there is value in looking at patterns of water quality changes at specific stations, as opposed to lumping all stations together in a larger area. Doing so, they assert, better highlights patterns of eutrophication, and can help to pinpoint areas in need of effective restoration measures, such as specific canals in central and northern Biscayne Bay.

Source: Millette, N.C., C. Kelble, A. Linhoss, S. Ashby, L. Visser.  2019. Using Spatial Variability in the Rate of Change of Chlorophyll a to Improve Water Quality Management in a Subtropical Oligotrophic Estuary. DOI: 10.1007/s12237-019-00610-5

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Restored vs. Natural Marsh: Is It All the Same to Nekton?
Analysis shows lower nekton density in restored marshes

In the northern Gulf of Mexico, 95% of commercially-caught fish depend on nearby coastal marshes at some point in their life stages. Yet these marshes, which make up about 40% of all coastal wetlands in the United States, are being lost at an alarming rate. As marsh restoration efforts seek to shore up these important habitats, a new meta-analysis investigated the relative value that restored marshes provide to nekton as compared to natural marshes. Individual studies of this question have, so far, yielded variable results.

The meta-analysis found that restored marshes tend to see a very gradual increase in relative density of nekton, reaching about 50% of natural reference sites in marshes less than 5 years after restoration, and 73% for projects greater than 5 years old. Total nekton and crustacean densities within restored marshes approached that of natural marshes after about 13 years. Fish densities, meanwhile, were more variable, but relatively comparable to reference sites regardless of age. The meta-analysis found that overall nekton density in restored marshes was still lower than that of a natural marsh, even after up to 30 years.

The authors offer several potential reasons that restored marsh nekton densities do not recover to the level of a natural marsh, including physical differences such as higher elevations and lower flooding duration in restored areas. They also suggest that this could be tied to the low organic matter found in dredged soils used to restore marshes, which could mean it likely takes a long time for organic matter -- vital to nekton production -- to accumulate in substrate. Given the highly variable results this research can yield, the authors recommend carefully selecting restored and reference sites that are equivalent in environmental conditions to measure restoration success.

Source: Hollweg, T.A et al. 2019. Meta-Analysis of Nekton Recovery Following Marsh Restoration in the Northern Gulf of Mexico. DOI: 10.1007/s12237-019-00630-1

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