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COI Funded Project: Eutrophication in Waquoit Bay: Effects on Visual Predation

Project Duration: 6/1/00-12/31/01

Proposed Research

Eutrophication (high nutrient loading) is a common ecological problem in estuaries located near populated areas, due to the leakage of nitrogenous waste into the watershed. Recent research has shown that eutrophication has profound effects on the ecology and species composition of estuarine ecosystems. Because estuaries are highly productive and important nurseries, their ecology affects the ecology of coastal waters, including, but not limited to, the populations of economically important fish and shellfish. Little work has been done on the effects of eutrophication on visual predation, despite the fact that the associated phytoplankton blooms can have dramatic effects on underwater light levels and visibility, particularly in the ultraviolet range. We propose to measure the optical effects of eutrophication in three estuaries with differing nitrogen loads in Waquoit Bay on Cape Cod, a site of concentrated ecological research. In addition we will measure the optical characteristics of common prey items, and, using established models, estimate the effects of eutrophication on visual predation. Because recent research suggests that the mummichog Fundulus heteroclitus (a key predator in Waquoit Bay) can see ultraviolet light, we will examine the effects on ultraviolet visual predation particularly closely. This research will provide an additional viewpoint on the ecological effects of eutrophication.

Project Report

Over the course of this project in 2001, approximately 400 measurements of downwelling irradiance were taken at thirteen different locations in the Waquoit Bay National Estuarine Research Reserve (Figure 1).   Measurements were taken at six different depths (surface and 20, 40, 60, 80, and 100 cm depth) on six different days chosen to span the plankton bloom that occurs in late August (July 9, 20, and 31; August 22; September 5 and 18).   The field spectroscopy system and sampling methods developed for this project worked admirably, resulting in a large database of high resolution (~0.3 nm) spectral data over a wide wavelength range (350-700 nm).

The measurements, in addition to providing valuable baseline data for this reserve, also resulted in a number of conclusions. First, there was a tremendous range of water transparency within the estuary. The clearest water was found in the bay itself, and the murkiest was found in the two estuaries that drain the most populous regions - Child's and Quashnet rivers. Hamblin and Sage Lot ponds had intermediate water transparencies. Second, the variability of water transparency was far higher at ultraviolet wavelengths than at visible wavelengths (Figure 2). This may have ecological implications, particularly since some of the fish in the reserve can see at ultraviolet wavelengths (e.g. killifish and silversides). The distribution of larvae and eggs near the surface, organisms particularly vulnerable to UV radiation, may also be affected by this variability.

Finally, there was also a great deal of temporal variability in water transparency at all the sites (Figure 3). Most of this variability followed the same basic pattern, with relatively high transparency in July and September straddling a time of low transparency in August.

Determining the relative effects of this variability on visual predation depends a great deal on how it is analyzed. Generally, the largest changes in attenuation coefficient (how water transparency is often measured) occurred at UV wavelengths and in the murkier waters, such as in the Child's river. However, because the large variability in the coefficients co-occurs with large average levels of these coefficients, this variability may not have much on the more ecological relevant factors of sighting distance and the penetration of damaging radiation. Briefly stated, if the water is so murky that the sighting distance is only 1 cm, doubling the murkiness (and reducing the sighting distance to 0.5 cm) may not make much difference, except for animals who's sighting distance is quite small. Figures 3a and 3b show the difference between the two forms of analysis.

The largest absolute changes in attenuation coefficient occur at the lowest wavelengths (Figure 3a), but the largest absolute changes in light transmission occur at approximately
580 nm (yellow light).

The relevant wavelengths then depend on scale of the sighting distances involved. For larger animals in the clearer regions of the reserve (such some of the larger fish species in the center of the bay itself) that generally have large sighting distances, the important variability is that in the yellow region of the spectrum. For smaller animals, which usually have shorter sighting distances, sighting distances at all wavelengths may be important. Therefore the high variability at lower wavelengths is more likely to affect smaller animals than larger ones.

In summary, water clarity in the Reserve has a tremendous amount of spatial and temporal variability, with plankton blooms likely causing the rapid decrease in the late summer. The variability is especially high at UV wavelengths, the ecological relevance of which likely depends on the size of the animals involved. I'd like to thank the Rinehart Coastal Research Center for its generous support and the Waquoit Bay National Estuarine Research Reserve for granting access and providing logistical support.

Sites for 2001 Waquoit Bay Study

Name        Location Coordinates
Child's River 1 below Edwards boatyard 41°34.781'N 70°31.740'W
Child's River 2 “Sound Bite” boat 41°34.547'N 70°31.875'W
Child's River 3 inside mouth of river 41°34.218'N 70°32.004'W
Quashnet River 1 fork between two docks 41°35.053'N 70°30.611'W
Quashnet River 2 below Meadow Neck Rd bridge 41°34.771'N 70°30.737'W
Quashnet River 3 inside mouth of river 41°34.580'N 70°30.862'W
Hamblin Pond 1 center of pond 41°34.446'N 70°30.338'W
Hamblin Pond 2 entrance of pond (green buoy) 41°34.222'N 70°30.367'W
Hamblin Pond 3 outside mouth of pond 41°33.592'N 70°30.924'W
Sage Lot Pond 1 back of pond 41°33.239'N 70°30.484'W
Sage Lot Pond 2 center of pond 41°33.372'N 70°30.463'W
Sage Lot Pond 3 inside mouth of pond 41°33.335'N 70°30.627'W
Waquoit Bay YSI monitoring site 41°34.207'N 70°31.169'W

Originally published: January 25, 2005