Ecologic and precursor success criteria for south Florida ecosystem restoration

A Science Sub-Group Report to the Working Group of the South Florida Ecosystem Restoration Task Force

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CHAPTER 6:

IMPROVEMENT OF WATER QUALITY IN ESTUARIES


Team

Leader: R. Stumpf, USGS.
Team Members: J. Fourqurean, M. Durako, C. Thomas.

Rationale

The selection of success criteria for improvement of water clarity is based on a strategy using the environmental significant of clear water. The clarity of the water has a direct impact on benthic communities, seagrasses in particular, that require high light levels. The amount of material in the water column may also affect benthic communities by smothering as sediment settles out of the water column. Turbidity in the water is driven by two factors: algal blooms and sediment. Increases in either of these result in increased turbidity and therefore a reduction in water clarity.

Recommended Specific Success Criteria

As the primary concern of improved water clarity is to improve the habitat for seagrasses, we recommend that light levels required for seagrass growth and regeneration where seagrasses have grown historically be the success criteria. However, we note that anthropogenic sources of significant turbidity have not been identified, if any exist. The primary source of suspended sediment in the Bay is resuspension by wind. the source of nutrients supporting algal blooms has not been well-defined, and may be local to the Bay. If these are natural causes, it may not be possible to manage directly for turbidity. Reduction of turbidity may therefore involve an indirect effect of other changes in the bay, such as increasing seagrass cover.

Reference Baseline

Explanation of Terms.

Currently water clarity or turbidity is described using one of the following: light attenuation, Secchi disk depth, nephelometry. Light attenuation is determined by determining the available light at different depths in the water, then determining the rate of loss from regression based on Beer's Law in the form:

K = ( -1/z) ln[L(z)/L (z0)]

where K is the attenuation coefficient, Z is the depth, L(z) is the light at depth z, and z0 is a reference depth. The regression is

ln [L(z)] = -Kz + constant

where K is the slope of the regression line and normally has units of m1. Smaller K means clearer water. The attenuation coefficient is used to calculate the amount of light available at a particular depth. If the depth is 1 m and K is 1 m-1, 36% of the light will reach the bottom.

Secchi disk depth gives a measure of visibility in the water. The Secchi disk depth corresponds approximately to 1/K. This relationship applies best when pigments, such as algal chlorophyll are found in the water. The carbonate sediments in Florida bay are highly reflective; this means that abundant sediment in the water can produce low visibility and a low Secchi depth, yet still allow significant light to reach the bottom, resulting in a low attenuation coefficient.

Nephelometry gives a measurement of the amount of sediment in the water, and provides relatively little information on the light attenuation by algae.

Conditions in the Bay.

Extremely limited data sets on light availability in Florida Bay are available from before 1990. The available data and anecdotal information indicate that through the 1980's water clarity was "extremely high" throughout the Bay. People working in the Bay at the time describe "gin-clear" waters, and the bottom was easily visible (i.e., Secchi disk depths exceeded the water depth). some light attenuation measurements are available from some seagrass studies, however these are not enough to fully describe conditions throughout the Bay.

Conditions have deteriorated over the last 5 years. Secchi disk depths are often less than the water depth. Light attenuation coefficients have been measured at >1.0 m-1.

An attenuation coefficient of 1 m-1 means about 36% of the light reaches the bottom in one meter of water, but only 14% reaches the bottom in 2 m of water. In Tampa Bay the EPA National Estuarine Program has developed a management strategy based on a determination that light levels of 20.5% must be present at the bottom for seagrass to grow. It has been suggested that light levels of >30% may be required when seagrasses are stressed, such as by sedimentation or epiphytic growth. We should note that for extremely shallow water, such as Florida Bay, a 5% difference in light level at the bottom is at the limit of the ability to measure attenuation (+/- 0.1 m-1). Examples of attenuation for depths in Florida Bay appear in Table 1.

Other measures such as Secchi Disk depth and nephelometric units are somewhat less reliable as measures of light attenuation. The carbonate sediments in Florida Bay are highly reflective and extremely effective at scattering light within the water column. Thus, even when the visibility is poor, light may be reaching the bottom. When sediments are found in the water, small Secchi depth may still correspond to conditions of low light attenuation. When algal blooms exist, however, small Secchi depths should correspond to high attenuation. Secchi depth and, particularly, nephelometry are good indicators of the amount of sediment in the water.

Table 1. Light Attenuation for 20% or 20% light to reach depths observed in Florida Bay.
light
%
depth
m
attenuation
(K) m-1
1/K
m

2011.60.625
2020.81.25
2030.531.87
2011.20.83
2020.61.67
2030.42.5

Recommendations

Continued measurements of light in Florida Bay are required to monitor for water clarity or turbidity. These measurements should involve determination of light attenuation, Secchi disk depth and sediment loads.

Currently light attenuation is measured on a sporadic basis. This should be more systematic, at the least coordination should be made between the various researchers making these measurements to assure adequate temporal and spatial coverage. Most researchers working in the Bay are using Licor spherical sensors, so the measurements should be comparable.

Secchi disk measurements are made by most programs. These provide the most comprehensive data set on the Bay. Additional examination of the relationship of Secchi disk depth to light attenuation will need to be explored. It may be possible to establish relationships by basin, as some basins are prone to algal blooms, and others tend to have sediment resuspension events. However, it is unlikely that a consistent relationship between Secchi depth and K will apply throughout the Bay.

Nephelometric measurements and suspended sediment measurements should continue in order to obtain better date on sediment loads. Nephelometry can provide a good measure of sediment concentrations with proper validation. As suspended sediment may have a direct bearing on the health of benthic communities, these variables may provide ancillary data on environmental conditions.

Remote sensing can be used to approximate light attenuation and sediment loads. These measurements can provide sufficient spatial and temporal data to interpolate the field programs. Current and past satellite data sets have some limitations, but allow a reasonable quantification of light attenuation and sediment concentration. However, manual adjustment of the data may be required for certain basins and conditions (particular basins that have strong algal blooms). When an ocean color satellite is launched, reliable quantification of light attenuation may be achievable under all conditions observed in the Bay.

Reports on historical and recommended seagrass distributions should be used in order to establish water clarity values. In addition, appropriate water clarity should be determined as a function of basin and water depth.

References

Kenworthy, W.J., and D. E. Haunert (eds.) 1991. The Light Requirements of seagrasses. Proceedings of a workshop to examine the capability of water quality criteria, standards and monitoring programs to protect seagrasses. NOAA Technical Memorandum NMFS-SEFC-287.

Janicki, a. and D. Wade. 1996. Estimating critical nitrogen loads for the Tampa Bay Estuary: an empirically based approach to setting management targets. Report to the Tampa Bay National Estuary Program # 94-444-14. March 14, 1996.


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