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
December 2009
Contents
A Decade of Eutrophication in the Skidaway River Estuary: More Small Phytoplankton, Less Oxygen, Big Food Web Changes
Passive Restoration of Former Agricultural Sites Can Bring Back Marsh Channel Networks
Nutrient Enrichment Increases Food Web Complexity in Florida Bay
Massive Tideland Reclamation Project + Monsoon Conditions = Sediment and Water Quality Impacts
A Decade of Eutrophication in the Skidaway River Estuary: More Small Phytoplankton, Less Oxygen, Big Food Web Changes
Some estuaries are lucky enough to be located near major oceanographic institutes, making them convenient sites for long-term monitoring. One example is the Skidaway River Estuary, GA, where researchers from the Skidaway Institute of Oceanography have collected a decade’s worth of data on nutrient and oxygen concentrations and phytoplankton community composition from the institute’s dock, revealing interesting patterns in food web dynamics related to eutrophication there. At approximately weekly intervals for ten years (1986-1996) , researchers collected data on nutrient concentrations, dissolved oxygen, chlorophyll a, phytoplankton abundances, and other physical and biological parameters. In addition, zooplankton tows were conducted every six weeks. This extensive data set enabled the researchers to draw conclusions not only about eutrophication, but also about changes in food web dynamics.
Likely because of increasing development around the estuary, nutrient loadings and eutrophication increased over the decade of observations. The investigators found that all planktonic groups except diatoms also increased over the course of the study, including bacteria and copepods. Diatoms did not increase, despite an increase in silicate. All planktonic groups except copepods also exhibited increasing annual amplitudes in seasonal cycles of abundance, with higher peak concentrations observed each year. The relative and absolute increase of small nanoplankton abundance has significant ramifications for food webs of the estuary: food webs that support higher production of fishes typically have diatoms at their base. Eutrophication also appears to have encouraged an augmentation of microbial food webs in the system.
Considering that the development trajectory in this watershed points toward continued nutrient loading, it is highly likely that other impacts on the estuary will follow, possibly including harmful algal blooms and alterations of higher orders of the estuarine food web. Long-term examination of all components of the food web here, as in other systems, is crucial.
Source: Verity, P. G. and D. G. Borkman. 2009. A decade of change in the Skidaway River Estuary. III. Plankton. Estuaries and Coasts 32(November 2009). DOI: 10.1007/s12237-009-9208-2.
Passive Restoration of Former Agricultural Sites Can Bring Back Marsh Channel Networks
The network of channels and pools that wind through salt marsh surfaces are essential to marsh functions, providing feeding areas for fish and birds and access onto the marsh surface for many organisms. Marsh vegetation also enters the detrital food web via the physical pathways provided by marsh channels. But these winding channels and standing pools were often drained when marshes were converted to agricultural lands. In many places in the world, marsh “reclamation” projects continue, constructing dikes and adding grids of straight ditches to enable agriculture. What happens when those agricultural sites are no longer in use, and are left alone to revert to marshland?
In the Bay of Fundy, it seems that much of that structure is restored, according to a recent study. Recovering and reference marshes in the bay were mapped to determine channel densities and sinuosity, and marsh pool area. Investigators found that the former agricultural sites exhibited recovery of extensive channel networks, often incorporating old drainage ditches into the networks. Overall, channel densities (total length of channels per unit marsh area) were similar in recovering and reference marshes. Because of the inherited drainage ditches, channels in recovering marshes tended to be straighter than in reference marshes. These recovering sites were also shown to have a higher degree of densely concentrated channel rivulets compared to reference marshes. In their study of tidal pools, the researchers found a greater area of pools on the recovering marsh, providing increased benefits to wading birds and waterfowl. This study demonstrated that attempts to restore agricultural marshes have excellent prospects without the extensive investment required for creation of new marshes.
One caveat: the authors discovered that aerial photography underestimated the number of channels and pools substantially (by 50%, in the case of channels) because many of the smaller ones were overlooked. Using aerial photos that are not ground-truthed might therefore be problematic in studies of this type.
Source: MacDonald, G. K., P. E. Noel, D. van Proosdij, and G. L. Chmura. 2009. The legacy of agricultural reclamation on channel and pool networks of Bay of Fundy salt marshes. Estuaries & Coasts 32(November 2009). DOI: 10.1007/s12237-009-9222-4.
Nutrient Enrichment Increases Food Web Complexity in Florida Bay
One way to study impacts of high nutrient loadings on food webs is to compare trophic dynamics in enriched and unenriched sites, but since enriched sites typically experience additional anthropogenic impacts (fishing pressure, upland development, etc.), results gained using this approach can be difficult to interpret. Another approach is to use in situ manipulations, enriching experimental sites with nutrients and comparing the parameters of interest before and after the additions. This approach was taken by researchers in Florida Bay, who wanted to examine the impact of increased nutrient availability on food web structure. Two sites, one eutrophic and one oligotrophic, were enriched with additions of N and P. Seagrasses and benthic epifauna were collected at each site before and after enrichment, and food web structure was examined using stable isotope analysis.
The effect of the nutrient additions on food web structure depended on the prior nutrient status of the site. Changes were small at eutrophic sites, but at low-nutrient sites, crustacean epiphyte grazer abundance increased and the diets of these grazers diversified, as did diets of benthic predators. The diets of benthic grazers shifted with nutrient enrichment from green macroalgae to an opportunistic seagrass (Halodule wrightii) that occurred only in the nutrient addition treatments. Food chain length appeared to be unaffected by site or nutrient treatment, but food web complexity increased at enriched sites. In this system, control seems to be more bottom-up than top-down.
This study sheds light on how food webs in seagrass-dominated sites will respond to nutrient enrichment, and emphasizes that the outcome depends on the initial nutrient status of the site.
Source: Armitage, A. R. and J. W. Fourqurean. 2009. Stable isotopes reveal complex changes in trophic relationships following nutrient addition in a coastal marine ecosystem. Estuaries & Coasts 32 (November 2009). DOI: 10.1007/s12237-009-9219-z.
Massive Tideland Reclamation Project + Monsoon Conditions = Sediment and Water Quality Impacts
Saemangeum Lake in Korea, formed by placement of a 33 km dyke across two rivers discharging into the Yellow Sea, is one of the biggest tideland reclamation projects in the world. Designed to create 283 km2 of new land and a 118 km2 freshwater reservoir, a study of water quality in the lake before and after construction of the dyke revealed that dyke construction led to drastic changes in sediment quality. After dyke completion, tidal range decreased from 6 m to less than 1 m, even when the sluice gates are fully open. The proportion of silt and clay in surface sediments appears to have increased significantly. Given that fine sediments are often associated with higher concentrations of organic carbon, total N, and total P, the authors suggest that concentrations of these nutrients have likely increased as well. A simple mixing model demonstrated that benthic fluxes of nutrients increase during the warm, wet monsoon season, when strong salinity stratification develops and temperatures and groundwater discharge are elevated. The authors infer from these results that nutrients are being released from the sediments during times of intense stratification, perhaps because stratification facilitates anoxic conditions that promote breakdown of organic material in the sediments.
The authors conclude that the elevation of benthic fluxes is attributable to a confluence of factors that are mutually reinforcing: increased stratification, temperatures, and groundwater discharge during monsoon season, and the increase in organic-rich sediments after dyke completion. The reservoir of organic material in the lake’s sediments must be managed carefully in order to restore sediment and water quality in this system, and in such reclamation projects in general. Caution is especially warranted because a similar Korean tideland reclamation project designed to create a freshwater coastal lake failed due to severe water quality degradation after construction of the dyke. That project was abandoned, and as a result, nationwide concern has arisen over Saemangeum Lake, which is twice the size of the previous failed project.
Source: Kim, S.-H., K. Kim, M. Lee, H.-J. Jeong, W.-J. Kim, J.-G. Park and J. Yang. 2009. Enhanced benthic nutrient flux during monsoon periods in a coastal lake formed by tideland reclamation. Estuaries & Coasts 32(November 2009). DOI: 10.1007/s12237-009-9217-1.
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