Lake Ecosystem Health Assessment: Indicators and Methods

Abstract

A set of ecological indicators including structural, functional, and system-level aspects were proposed for a lake ecosystem health assessment, according to the structural, functional, and system-level responses of lake ecosystems to chemical stresses including acidification, eutrophication, and copper, oil and pesticide contamination. The structural indicators included phytoplankton cell size and biomass, zooplankton body size and biomass, species diversity, macro- and micro-zooplankton biomass, the zooplankton/phytoplankton ratio, and the macrozooplankton/microzooplankton ratio. The functional indicators encompassed the algal C assimilation ratio, resource use efficiency, community production, gross production/respiration (i.e. P/R) ratio, gross production/standing crop biomass (i.e. P/B) ratio, and standing crop biomass/unit energy flow (i.e. B/E) ratio. The ecosystem-level indicators consisted of ecological buffer capacities, exergy, and structural exergy. Based on these indicators, a direct measurement method (DMM) and an ecological modeling method (EMM) for lake ecosystem health assessment were developed. The DMM procedures were designed to: (l) identify key indicators; (2) measure directly or calculate indirectly the selected indicators; and, (3) assess ecosystem health on the basis of the indicator values. The EMM procedures were designed to: (l) determine the structure and complexity of the ecological model according to the lake's ecosystem structure; (2) establish an ecological model by designing a conceptual diagram, establishing model equations, and estimating model parameters; (3) compare the simulated values of important state variables and process rates with actual observations; (4) calculate ecosystem health indicators using the ecological model; and, (5) assess lake ecosystem health according to the values of the ecological indicators. The results of a case study demonstrated that both methods provided similar results which corresponded with the lake's actual trophic state.

Introduction

During the last decades, the phrase “ecosystem health” has been used with increasing frequency in the literature. With increasing frequency, environmental managers have begun to consider the protection of ecosystem health as one of their primary goals in environmental management. Since the late 1980s, the International Society for Aquatic Ecosystem Health and Management (ISAEHM), the International Ecosystem Health Society (IEHS), and the international publications Aquatic Ecosystem Health and Management and Ecosystem Health have made significant contributions to the theoretical development of this new field. A number of different definitions (e.g. Karr et al., 1986; Schaeffer et al., 1988; Rapport, 1989; Kay, 1991; Norton, 1992; Costanza, 1992; Haskell et al., 1992; Page, 1992; Ulanowicz, 1992) and indicators (e.g. Hannon, 1985; Rapport et al., 1985; Karr et al., 1986; Ulanowicz (1986), Ulanowicz (1992); Kay and Schneider, 1991; Costanza, 1992; Jørgensen (1995a), Jørgensen (1995b)) have been proposed and discussed in the literatures. Costanza, in 1992, summarized the conceptual attributes comprising the definition of ecosystem health as follows: (1) homeostasis; (2) absence of disease, (3) diversity or complexity; (4) stability or resilience; (5) vigor or scope for growth; and, (6) balance between system components. He went on to emphasize that it was necessary to consider all or at least a majority of the definition's attributes simultaneously in order to achieve a viable working understanding.

Historically, ecosystem health was measured using indices for a particular species or other components of the system. Many of those efforts, however, were found to be somewhat restrictive or lacking in terms of completeness of the system as a whole. In order to better reflect ecosystem complexities, a number of new indices have been proposed, e.g. gross ecosystem product (GEP) (Hannon, 1985), ecosystem stress indicators (Rapport et al., 1985), the index of biotic integrity (IBI) (Karr et al., 1986), the index of network ascendancy (Ulanowicz (1986), Ulanowicz (1992)), the overall system health index (HI=V×O×R) (Costanza, 1992), and exergy, structural exergy, and ecological buffer capacities (Jørgensen (1995a), Jørgensen (1995b))). Clearly, the different indicators cover different aspects of ecosystem health. A more complete image, however, of the conditions behind ecosystem health may require the simultaneous application of several such indicators (Jørgensen, 1997). This seems further supported given the breadth in scope of ecosystem health as a concept. Certainly it encompasses not only biophysical dimensions, but also social, economic, and human aspects as well (Karr, 1992; Rapport, 1995).

Several procedures for ecosystem health assessment have been proposed. Schaeffer et al. (1988) suggested seven guidelines. Haskell et al. (1992) suggested the following sequence for those guidelines: (1) identifying symptoms; (2) identifying and measuring vital signs; (3) making a preliminary provisional diagnosis; (4) conducting tests to verify the diagnosis; (5) making a prognosis; and (6) prescribing treatment. Jørgensen (1995a) suggested a tentative procedure for the practical assessment of ecosystem health where the ecological indicators (exergy, structural exergy, and buffer capacities) would be used to: (1) identify the relevant questions related to the health of the ecosystem under consideration; (2) assess the most important mass flows and mass balances related to these questions; (3) make a conceptual diagram of the ecosystem, including the components most important for the mass flows defined in (2); (4) develop a steady state or dynamic model using the conventional procedure (Jørgensen, l994); (5) calculate the exergy, structural exergy, and relevant buffer capacities using a model; and, (6) assess the ecosystem health according to the values of exergy, structural exergy, and buffer capacities obtained from the model.

The objectives of this paper are: (1) to propose a set of comprehensive ecological indicators including structural, functional, and system-level aspects to be used in lake ecosystem health assessments; (2) to propose two methods for lake ecosystem health assessment; and, (3) to apply the suggested ecological indicators and methods to the ecosystem health assessment of a Chinese shallow lake (Lake Chao).