Prepared by the University of Connecticut and the Connecticut DEP, with the support of researchers and organizations throughout the Long Island Sound watershed.
Establishing restoration objectives for eelgrass in Long Island Sound

Literature Reviews

Click to download the Seagrass Literature Survey Report - PDF Document

Click to download the Case Study Report - PDF Document

Click to download the Quality Assurance Protection Plan - PDF Document

Summary - Seagrass Literature Survey

Eelgrass (Zostera marina) was once prevalent throughout the shallow coastal areas of Long Island Sound (LIS). The Atlantic-wide die off of the species in the 1930s resulted in the loss of eelgrass from much of the local area, but healthy populations were reestablished in the eastern portion of LIS by the 1950s. The recovery of eelgrass in the western Sound was less successful and today, those populations have vanished. Since the 1950s, eelgrass populations along the Connecticut coast have suffered additional losses believed to be linked to the effect of nitrogen loading on the coastal ecosystem. The aim of this report was to summarize the literature regarding the factors affecting the growth and distribution of Z. marina relevant to Long Island Sound and identify levels for water quality standards and habitat guidelines which would be protective of Z. marina. Part II of this report presents the application of the water quality recommendations to three case study sites (Niantic River, Mumford Cove, Pawcatuck River / Little Narragansett Bay), including verification of the recommended habitat guidelines and a comparison of the nitrogen loads to various Connecticut estuaries.

The most important factor governing both the distribution and growth of Z. marina is the availability of light. The maximum depth of distribution is governed by the minimum amount of light required by Z. marina, which is about 20% of the surface light if measured as light in the water column. In addition to being attenuated by the water, light is also attenuated by particles in the water (phytoplankton, total suspended solids) and by epiphytes on the leaves. Measuring the light in the water column via Secchi depth or a light meter (light attenuation coefficient) accounts for the particles in the water. To account for the light attenuated by the epiphytes, direct measurements of the epiphytes are required or a model may be employed which relates water column characteristics to an estimate of epiphyte biomass. If the light attenuated by epiphytes is taken into account, the minimum light required by Z. marina should be around 15% of the surface light. Free-floating macroalgae may also shade Z. marina if it becomes abundant. In some New England estuaries, macroalgae has completely overgrown Z. marina to become the dominant primary producer.

The minimum depth is governed by the tidal range and the degree of wave action experienced in the area. Z. marina, in general, requires a depth greater than the tidal amplitude in order to avoid exposure during low tide. For Long Island Sound, it appears a vertical distance of 1m is needed between the minimum and maximum depth limits for a bed to flourish.

Temperature and nutrients may affect Z. marina indirectly or directly. Temperature and nutrients can affect light availability by stimulating or suppressing the growth rate of epiphytes, phytoplankton, or macroalgae. Nitrogen can directly affect Z. marina by stimulating productivity, if nitrogen is limiting. However, light is usually the limiting factor in LIS. When nitrogen availability is high, the N can become toxic to the plant by initiating the over utilization of carbon which should instead be stored for later use by the plant. Z. marina exhibits optimal temperature ranges for growth and photosynthesis. The water temperature affects the distribution, and the annual and seasonal variability within beds.

While light is the main factor controlling eelgrass distribution and growth, with nutrients and temperature indirectly and directly affecting the autoecology of Z. marina, other features of the habitat also determine whether eelgrass will successfully colonize a particular location. These features include physical aspects of the sites (tide, waves, current speed), sediment characteristics (percent organics, sediment sulfides), and water column characteristics (oxygen, salinity). Eelgrass exhibits a range of tolerance for each of these factors. For the physical factors, leaving the range of tolerance results in a physical disturbance to the plant such as burial or uprooting. For the sediment and water column characteristics, leaving the zone of tolerance typically results in a physiological response, a translocation of internal resources. The plant can deal with short term excursions outside the zone of tolerance, but extended periods of exposure to unfavorable conditions typically results in a degradation of plant tissue.

Restoration guidelines for submerged aquatic vegetation based on water quality and habitat-based requirements have been developed for the Chesapeake Bay region by evaluating decades of monitoring data, experimental evidence, statistical analyses of the data, and modeling efforts (Batiuk et al. 1992; Batiuk et al. 2000). These guidelines have been developed to include marine and freshwater submerged aquatic vegetation (SAV). The Chesapeake Bay guidelines for Z. marina were examined relative to a recent study looking at habitat requirements for Z. marina in Long Island Sound (Yarish et al. 2006) and data from the three case study sites presented in Part II of this report.

Table 1: Recommended habitat requirements for the growth and survival of eelgrass.

The restoration guidelines were presented in terms of the percent light received by the plant, either at the leaf surface (which includes attenuation by the epiphytes) or through the water column. Light in the water column was designated as the primary requirement, meaning it was the primary factor determining whether Z. marina was found in a particular location. The secondary requirements (nutrients, chlorophyll-a, total suspended solids) affected the availability of light and may have directly affected the physiology of the plants. Both the primary and secondary requirements are water quality based metrics with the potential for change under management.

The factors listed as "habitat constraints" were related to the physical and sediment characteristics of the habitat. The physical factors (current velocity, minimum depth of distribution) helped to identify whether a certain area was suitable for eelgrass, but these factors were not likely to be changed due to mitigation efforts. The maximum depth of distribution, and thus the vertical distribution, could change as a result of changing water quality. The sediment characteristics should also change as a result of changes in the water quality or primary producer community. But these habitat constraints were used primarily as a means of explaining why Z. marina was not present in a location where the water quality appeared suitable.

Suggestions for future research included:

References Cited in Lit. Summary - see Seagrass Literature Survey Paper for a full list

Batiuk, R. A., P. Bergstrom, M. Kemp, E. W. Koch, L. Murray, J. C. Stevenson, R. Bartleson, V. Carter, N. B. Rybicki, J. M. Landwehr, C. L. Gallegos, L. Karrh, M. Naylor, D. Wilcox, K. A. Moore, S. Ailstock & M. Teichberg. 2000. Chesapeake Bay submerged aquatic vegetation water quality and habitat-based requirements and restoration targets: A second technical synthesis. Annapolis, Maryland: United States Environmental Protection Agency. Report for the Chesapeake Bay Program. report number CBP/TRS 245/00 EPA 903-R-00-014.

Batiuk, R. A., R. J. Orth, K. A. Moore, W. C. Dennison, and J. C. Stevenson. 1992. Chesapeake Bay submerged aquatic vegetation habitat requirements and restoration targets: A technical synthesis. Virginia Inst. of Marine Science, Gloucester Point (USA). Report. report number CBP/TRS-83/92.

Yarish, C., R. E. Linden, G. Capriulo, E. W. Koch, S. Beer, J. Rehnberg, R. Troy, E. A. Morales, F. R. Trainor, M. DiGiacomo-Cohen & R. Lewis. 2006. Environmental monitoring, seagrass mapping and biotechnology as means of fisheries habitat enhancement along the Connecticut coast. Stamford, Connecticut: University of Connecticut. Final Report submitted to the Connecticut Department of Environmental Protection, Hartford, CT. report number CWF 314-R.

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Summary - Nitrogen in Long Island Sound

A comparison of the estimated nitrogen loads to the three systems shows that Niantic River received a total N-load greater than Mumford Cove (figure 1). However, when normalized to the area of the estuary, the N-load to Niantic River was similar to that of Mumford Cove. Pawcatuck River had the largest N-load, both in terms of total load to the system and normalized to the area of the estuary. The N-load to LNB was a rough estimate, as this value had not been calculated in the datasets used. The value for LNB alone was from the portion of the watershed draining into LNB. The higher load was estimated by summing the values for LNB, Wequetequock Cove and Pawcatuck River. The true value likely lies somewhere between these two estimates. The N-load values obtained for the estimate of the Pawcatuck River (Branco and Kremer in prep) roughly agrees with other estimates for the system (Desbonnet and Lee 1996; Mullaney et al. 2002).

Figure 1: Comparison of the annual nitrogen loading rate for a number of Connecticut estuaries. Estimates were developed using the N-Load model, using the 2002 CLEAR land use data set. The line indicates the total nitrogen delivered to the system. The white bars indicate the size of the estuary. The gray and black bars indicate the nitrogen load delivered per square meter of the estuary, broken down by source. The values for LNB came from Desbonnet (1991) and only include the watershed draining into LNB. The actual value of the N-Load to LNB is likely between the LNB value and the value for "LNB+Wequetequock+Pawcatuck," which was the sum of the N-load from those three sites.

References Cited in Case Study Summary - see Case Study Paper for a full list

Branco, A. and J. N. Kremer. in prep. Predicting the nitrogen load to 10 Southern New England estuaries using a modified version of the N-LOAD nitrogen loading model.

Desbonnet, A. and V. Lee. 1996. Rhode Island Coastal System: Databases for Determining Nitrogen Sensitivity. Vol. 1. Text and Statewide Database. Vol2. Bay Profiles: Pawcatuck River to Narrow River. Vol. 3. Bay Profiles: Wickford to Seekonk River. Vol. 4. Bay Profiles: Bristol, Little Compton and The Islands. Final Report to the Rhode Island Department of Environmental Management, Narragansett Bay Project, Providence, R.I. report.

Mullaney, J. R., G. E. Schwarz, and E. C. T. Trench. 2002. Estimation of nitrogen yields and loads from basins to Long Island Sound, 1988-98. U.S. Department of the Interior, U.S. Geological Survey, Prepared in cooperation with the Connecticut Department of Environmental Protection. report.

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