The definition of tropic state I develop here is designed to include both autotrophic and heterotrophic components; thus, there is a ‘‘heterotrophic state’’ and an ‘‘autotrophic state’’ of a stream or river. Heterotrophic state can be defined as the metabolic activity of the stream (typically measured as average O2 demand [respiration, R] during dark periods and scaled to 24 h). Autotrophic state is the gross primary production (GPP) during lighted periods (typically measured as production and scaled to 24 h). The delineation of heterotrophic and autotrophic state in flowing waters was pioneered by Odum (1956). I propose that eutrophication in lotic habitats be defined as an increase in a nutritive factor or factors that leads to greater whole-system heterotrophic or autotrophic metabolism.
Heterotrophic state and autotrophic state are not mutually exclusive; a system with substantial autotrophic activity will likely have high heterotrophic activity and certainly have high respiration. This link between autotrophy and respiration can lead to a positive correlation between respiration and GPP (Fig. 1). But, a system with more heterotrophic activity does not necessarily have more autotrophic activity (e.g., the condition obtained with high BOD loading). Thus, GPP:R can indicate the balance between heterotrophic and autotrophic state. Considering both autotrophic and heterotrophic components accounts for enrichment by organic C in addition to N and P, and accounts for the observation that lotic food webs can be based on consumption of autotrophic or heterotrophic organisms.
My proposed definition of lotic trophic state is based on total heterotrophic and autotrophic production and influenced by emerging research on lakes. Although production of lakes has often been linked to planktonic biomass (usually expressed as chlorophyll concentrations), lakes can be net heterotrophic and highly influenced by terrestrial C inputs (e.g., Cole et al. 1994). Thus, solely emphasizing autotrophic biomass might not accurately describe trophic structure in lentic ecosystems. Rivers and streams are likely to be more dominated by heterotrophic processes than lakes given their stronger linkage to terrestrial systems as a source of organic C and the greater likelihood that light is intercepted. In small streams, the riparian canopy often shades the stream bottom, turbidity greatly attenuates light in many large, well-mixed rivers, and in some streams (blackwater streams), dissolved organic C colors the water and retards primary production. In many rivers and streams, much allochthonous organic matter enters seasonally and through storm water runoff. The net production of most streams is negative (i.e., GPP:R ,1), even in open-canopy, shallow, clear-water streams (Mulholland et al. 2001). Thus, any definition of eutrophication in streams should consider heterotrophic activity. Autotrophic activity can also be important in rivers and streams. Some streams with open canopies are net autotrophic (Mulholland et al. 2001). Phytoplankton production can supply a significant portion of the productivity in medium to large rivers that are not highly turbid and do not completely mix because they have zones with limited water replacement (e.g., Thorp et al. 1998; Wehr and Descy 1998). Thus, allochthonous and autochthonous sources of C both should be considered, as well as inorganic and organic forms of nutrients such as N and P, when defining trophic status of lotic ecosystems.
Historically, trophic state in lakes was defined on the basis of clear delineation between anoxic hypolimnia and oxygenated waters (i.e., the difference between a mesotrophic and a eutrophic lake) and subsequent increases in the prevalence of cyanobacterial blooms, eutrophication- resistant animals, decreased water clarity, and taste and odor problems. Foremost, biogeochemical processes favor increased internal loading of P, leading to a positive feedback that stabilizes the eutrophic state with an anoxic hypolimnion (Dodds 2002). Such clear delineation of eutrophic conditions does not occur in shallow lakes, wetlands, and lotic systems for a variety of reasons.
Rivers and streams are relatively shallow and have considerably greater rates of atmospheric exchange compared with lentic systems, except under very low flow conditions when they become similar to small, shallow lentic systems. Thus, it is difficult for biota to consume all the O2 in the water column without substantial inputs of BOD and adequate nutrients to support very rapid rates of heterotrophic activity. Anoxia is rare in the water column of natural rivers and streams, even in forested streams under deciduous canopies immediately after leaf fall. In most lotic systems, internal loading of P and N tends to be dominated by remineralization, groundwater inputs, and erosion. Subsequently, alternative methods are required for describing trophic distributions in lotic ecosystems.
An approach that uses statistical distribution of benthic chlorophyll and water column nutrients was proposed to classify trophic state in streams given a lack of breakpoints (Dodds et al. 1998). Trophic categories by statistical distributions signify the probabilities of each trophic state. However, Dodds et al. (1998) used distributions from data sets that included affected sites; thus, the proposed categories do not represent natural trophic distributions. Many regions of developed countries completely lack such reference sites. However, a broad definition of stream trophic state requires consideration of the historical condition of streams before substantial modification that might influence heterotrophic or autotrophic state.