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2. PROBLEM STATEMENT

Thurston Pond is a small, shallow, water body (Figure 2). Such ponds in the Midwest typically exist in one of two distinctly different water clarity states, clear or turbid. In the small, clear-water ponds, the pond bottom is typically visible when viewed from above the water’s surface, whereas in small, turbid-water ponds, the pond bottom is usually not visible where the water is more than a few inches deep. In clear-water ponds, submersed, floating-leaved, and emergent aquatic plants dominate the vegetation; invertebrates (insects, zooplankton, and crustaceans) and fishes are present, and biodiversity is high. In turbid-water ponds microscopic and filamentous algae dominate the plant community, other vegetation is largely absent, and the invertebrate and fish communities are reduced to a few tolerant species or are absent.

2.1. Thurston Pond Characteristics: Yesterday and Today.

Thurston Pond was initially a healthy, clear-water pond, but over its 40-year life it has become a degraded, turbid-water pond. The observed characteristics for these two states are outlined in the following table.

Table 1. Characteristics of Thurston Pond under clear- and turbid-water states.
Characteristic	State
Characteristic	Clear-water (yesterday)	Turbid-water (today)
Maximum water depth	About 6.5 feet over hard clay pond bottom.	About 5 feet.
Sediment infilling	No infilling	In the northeast corner of the pond about 1.5 feet of soft, loose sediment has accumulated on top of the hard clay pond bottom.
Water supply	Major sources prior to 1972 were ground water and surface runoff from undeveloped lands within the Thurston Nature Center and surrounding area.	Major sources now also include storm water from street drains in the Orchard Hills subdivision and from Clague School property.
Water level	Initially increased by berm construction at west end of pond.	Decreasing due to sinking of berm and overflow of berm.
Water clarity	Pond bottom visible in spring throughout the pond; Secchi disk visible on bottom in spring at about 6.5 feet.	Pond bottom visible in spring in water about 1-1.5 feet deep. Secchi disk visible at 1.2-2.3 feet in spring and 0.4 feet in summer.
Water odor	None	Musky
Water color	None; clear.	Brownish or greenish in appearance due to microscopic algae and suspended particles of organic matter.
Dissolved oxygen	Adequate to support all desirable native aquatic life forms throughout the year.	Supersaturated during the day in summer. May be limiting at night during the summer, especially at pond bottom, and in winter under ice. Significant fish kill seen on ice-out in 2003.
Phosphorus	No data; probably low.	Phosphorus levels in water extremely high (0.41mg/L); no data for sediments, but probably higher than in water.
Nitrogen	No data; probably low.	No data; probably high.
Conductivity	No data.	350-500 microS/cm; problem level is >1000
Total dissolved solids	No data.	Approximately 250.
pH	No data.	About 7.4-9.8; values > 8.5 can be a problem.
Temperature	Suitable for native plants and warmwater animals.	Suitable for native plants and warmwater animals.
Algae	Low density; no obvious accumulations.	Major cause of low water clarity; may be floating mats of noxious filamentous species.
Rooted aquatic plants	Abundant and diverse.	Low density or absent.
Zooplankton	Unknown.	Low density or absent.
Aquatic insects	Abundant and diverse	Low density or absent
Fish	Introduced native fishes (bass, sunfishes, and minnows) abundant; bullheads and carp and goldfish absent or present in low abundance; species diversity reflects stocking history.	Low density, absent, or restricted to tolerant native species (bullheads) and exotic species (carp and goldfish).
Amphibians (Thurston Pond, adjacent wet woodland & vernal pond)	Frogs and toads (6 species), and newts and salamanders present (1 species each).	The Ann Arbor frog and toad survey reports low density, 4 species of frogs and toads present, but no newts or salamanders.
Reptiles	Snapping and painted turtles abundant.	Snapping and painted turtles abundant.
Ducks and geese	Mallard ducks and Canada geese abundant; diving ducks as regular spring visitors.	Mallard ducks and Canada geese very abundant; diving ducks less frequently seen.
Wading birds	Three species of herons plus bittern and egret commonly seen.	Great blue herons are commonly seen; egrets are infrequent visitors.
Raptors	Migrating ospreys occasionally seen fishing in the pond in the spring.	Ospreys rarely seen.
Muskrats	Common residents	Absent

The measurements for turbidity, phosphorous, pH, and total dissolved solids made in 2004 indicate some serious problems with water quality. The turbidity and phosphorous measurements translate to a Carlson Trophic Index of 90, on a scale from 0 to 100. This reflects a hyper-eutrophic system dominated by algae to the exclusion of a healthy and diverse community of aquatic plants and animals. Using the Mitchell and Stapp water quality index, these same four measurements taken together yield a value of 37, on a scale of 0 to 100, thus putting Thurston Pond water quality solidly in the “Bad” category.

2.2. Possible Causes for Poor Water Quality.

The factors causing Thurston Pond to degrade from the clear-water state to the turbid-water state cannot be unequivocally demonstrated because historical water quality data are lacking. However, in other similar shallow ponds at this latitude, the change is usually initiated by a human-mediated influx of nutrients from the surrounding watershed. These nutrients stimulate the production of algae blooms in early spring. If there is an insufficient population of zooplankton to “graze down” the enhanced algae population, the algae shades the pond bottom and inhibits or prevents the growth of rooted aquatic plants. If this condition persists for a sufficient number of growing seasons (years), the rooted plant community declines and eventually dies out. The decline of the rooted aquatic plant community makes the nutrients that would have supported their production available to algae, thus further stimulating algae production. The decline and loss of the rooted vegetation also adversely impacts the zooplankton that graze on and help suppress the algae, because these zooplankton typically seek refuge from fish in the stands of rooted plants. The excess production of algae in response to an increase in available nutrients can also contribute to turbidity problems if dead and decaying algal matter remains in suspension or is re-suspended by wind and wave action or by the activities of fish (particularly bullheads, carp, and goldfish) turtles, and waterfowl.

2.3. Impact on Community.

In addition to its role as an educational resource, Thurston Pond is also the focus of intense community use. In a 1998 poll of residents of the Orchard Hills neighborhood, 82% of the 211 respondents stated that they visit Thurston Pond on a regular basis. Unfortunately, only 43% stated that they were at least “satisfied” with the condition of the pond. While some wildlife does remain, and is greatly appreciated by residents, it is greatly decreased in amount and diversity from two or three decades ago.

2.4. Impact on Educational Opportunities.

As a Nature Center owned by the Ann Arbor Public School District, the mission of providing high-quality educational opportunities is a very important one for Thurston Pond. Elementary and other students often visit local ponds as part of the environmental education curriculum. Thurston Pond was a site for this field trip for many years, until several years ago when decreasing density and diversity of aquatic life led to removing Thurston from the list of usable sites. Thurston students are now bussed to other pond sites, at considerable expense, and Thurston classrooms have greatly reduced opportunities to observe a healthy pond system in their own backyard.

No data; probably low.

No data; probably high.