The many recent alterations in the structure of the Otsego Lake ecosystem are dealt with in greater detail in the rel evant sections under "Descriptive Ecology". Some reiteration is necessary to provide a holistic view of system-wide biodiversity, biological integrity and ecological sustainability.
By biodiversity we mean species richness plus some evaluation of dominance. Biological integrity compares present species diversity with an earlier (presumed closer to pristine) state. Sustainability is a measure of integrity, including consideration of the presence or absence of, or the dominance of, exoti c species. It is a measure of the ability of a community to maintain its ecological functions while resisting successful invasion by exotic species (Woodley et al., 1993).
Figure 119a illustrates the assumed direction of energy flow through abundant or dominant organisms in Otsego Lake during the 1970s. Figure 119b illustrates the current situation. Introductions of organisms during the 1980s and 1990s have, in part, resulted in the following significant changes:
1. The phytoplankton community is now commonly dominated by Oscillatoria, Fragillaria, Synedra, Melosira and Chlamydomonas (Ramsey, Unpbl.) (see "Phytoplankton" section). These genera have been cited as typical of eutrophic alkaline waters (Wetzel, 1983) in contrast to some of the historically dominant taxa, such as Dinobryon and Ceratium, typical of mesotrophic and oligotrophic waters (Harman et. al., 1980). This indicates a tendency towards increasing eutrophy.
2 . Myriophyllum spicatum, Potamogeton pectinatus, P. crispus and Heteranthera dubia are the most widespread macrophytes (Harman, 1994b). Myriophyllum spicatum and P. crispus are introductions. Between 1935 and 1993, the number of species of submergent macrophytes distributed throughout the littoral zone in Otsego Lake remained between 22 and 25. Through 1986, 4-6 species shared dominance including the macro-alga Chara vulgaris, which is noted for its intolerance to ph osphorus enriched substrates (Vertucci and Harman, 1978a; b; c). In 1993, only one species, the introduced Myriophyllum spicatum, was considered abundant. It currently dominates the entire littoral macrophyte community. Many littoral areas that were formerly occupied by C. vulgaris currently have no macrophytes present. Both changes indicate increasing eutrophy as well as a loss of ecological sustainability.
3. In the late 1960s, tubificid oligochaetes were dominant in sediments from 40 to 50 m in depth, while chironomid larvae made up most of the biomass between 4 and 40 m. A diversity of arthropods dominated the littoral substrates. In 1993, tubificids were abundant from 30-50 m in depth, whereas chironomids only dominated substrates from 20-35 m. The greatest biomass in the littoral regions was made up by several species of snails and fingernail clams (Harman, 1994c) (see "Zoobenthos" section).
Alterations in the eulittoral zoo-macrobenthic community have res ulted in a lake-wide reduction of species richness of 27.5%. The species richness of "pollution intolerant taxa", as represented by the Ephemeroptera (mayflies), Plecoptera (stoneflies) and Trichoptera (caddisflies), has been reduced by 56%, including the extirpation of stoneflies. The species richness of the eulittoral molluscan fauna has been reduced by 53%. Much of this loss is assumed to be attributed to changes in the level of the lake due to the renovation of a the Village dam in the early 1950 s and chronic eulittoral siltation (Harman, 1974c). Photographs from the 1920-30s illustrate a shoreline of much different character than that present today. Harman (1971b), discussing the distribution of Otsego Lake mollusks in 1969-70, mentioned the potential negative effects of increased siltation resulting from the creation of an artificial sand beach in the Hyde Bay area.
Changes in the deeper littoral regions, which have resulted in the loss of much of the Arthropod (Insect) biomass, a re assumed to be associated with decreases in the diversity of food and cover. This change presumably resulted from the reduction of macrophyte diversity due to the dominance by M. spicatum mentioned above. Changes in the distributions of the profundal fauna (oligochaetes and chironomids; see "Zoobenthos" section) correlate with recent decreases in late-summer oxygen concentrations in those areas (see "Dissolved Oxygen" section), which is a function of nutrient loading and increasing eutrophic ation.
4. Before 1990, the zooplankton community was dominated by large crustaceans. Concentrations far exceeded those found in the Finger Lakes to the west (Harman et. al., 1980). In recent years, the crustacean zooplankters of the limnetic zone have been dominated by small individuals of the cladoceran Bosmina and comparable numbers of cyclopoid and calanoid copepods. These, in turn, are subordinate to several taxa of rotifers which now dominate the zooplankton. Keratella and Polyartha are usually the most abundant (France and Taylor, 1994).
The drastic alterations experienced by the zooplankton community are correlated with the introduction of the alewife. This forage fish is an extremely effective predator of large crustaceans, such as Daphnia (Cladocera) and the larger copepods (Cyclopoidea, Calanoidea). Historically important cisco populations have crashed, probably due to direct competition with alewives (Frost, 1993). Released from co mpetition with the crustaceans, rotifer populations have increased rapidly (France and Taylor, 1994). Rotifers are not as effective in reducing algal populations as are crustaceans; therefore, algal standing crops have increased, contributing to turbidity and possibly causing further reduction in late summer, deep-water oxygen concentrations.
5. The fish community in Otsego Lake and its watershed, has undergone several important changes (Foster, 1996). Recent introductions include Atlantic salmon, brown trout, smelt, alewife, fathead minnow, fallfish and European rudd. American eels are declining due to barriers to migration in the Susquehanna River. Cisco, and possibly whitefish, are declining due to competitive interactions with the alewife. Emerald shiners, bridle shiners, blackchin shiners, spottail shiners, rosyface shiners, spotfin shiners and smelt, all schooling planktivores, appear to be in decline. Populations of tadpole madtoms, margined madtoms, redfin pickerel, bluegills, slimy sculpins, burbot and walleyes have declined, several drastically. The populations of white suckers are increasing, as are those of several cyprinids, including pearl dace, redbelly dace, redside dace and cutlips minnow. The above alterations in the fish community appear to be directly related to exotic introductions, most importantly the alewife, as well as increasing eutrophy and decreasing stream quality (Foster, 1996).
Populations of organisms in habitats experiencing gradually increasin g eutrophy will thrive until impacted by some limiting factor (Lindeman, 1942; Andrewartha and Birch, 1964). If such factors are density independent, but trophic driven, they will cause catastrophic destruction at some increased level of productivity. Although annual fish kills in small impoundments are classic examples, deoxygenation of the hypolimnion of the central basin of Lake Erie in the 1960s clearly indicates the occurrence of this phenomonen in large bodies of water supporting cold water fis heries (Burns, 1985). Otsego lake trout populations, which currently are thriving, coupled with increasing oxygen stresses, are illustrative of a somewhat earlier stage in this scenario.