Jonathan J. Cole

Trophic cascades revealed in diverse ecosystems

New studies are documenting trophic cascades in theoretically unlikely systems such as tropical forests and the open ocean. Together with increasing evidence of cascades, there is a deepening understanding of the conditions that promote and inhibit the transmission of predatory effects. These conditions include the relative productivity of ecosystems, presence of refuges and the potential for compensation. However, trophic cascades are also altered by humans. Analyses of the extirpation of large animals reveal loss of cascades, and the potential of conservation to restore not only predator populations but also the ecosystem-level effects that ramify from their presence.

Carbon dioxide supersaturation in the surface waters of lakes

Data on the partial pressure of carbon dioxide (CO2) in the surface waters from a large number of lakes (1835) with a worldwide distribution show that only a small proportion of the 4665 samples analyzed (less than 10 percent) were within �20 percent of equilibrium with the atmosphere and that most samples (87 percent) were supersaturated. The mean partial pressure of CO2 averaged 1036 microatmospheres, about three times the value in the overlying atmosphere, indicating that lakes are sources rather than sinks of atmospheric CO2. On a global scale, the potential efflux of CO2 from lakes (about 0.14 x 1015 grams of carbon per year) is about half as large as riverine transport of organic plus inorganic carbon to the ocean. Lakes are a small but potentially important conduit for carbon from terrestrial sources to the atmospheric sink.

Reconciling carbon-cycle concepts, terminology, and methods

Recent projections of climatic change have focused a great deal of scientific and public attention on patterns of carbon (C) cycling as well as its controls, particularly the factors that determine whether an ecosystem is a net source or sink of atmospheric carbon dioxide (CO2). Net ecosystem production (NEP), a central concept in C-cycling research, has been used by scientists to represent two different concepts. We propose that NEP be restricted to just one of its two original definitions—the imbalance between gross primary production (GPP) and ecosystem respiration (ER). We further propose that a new term—net ecosystem carbon balance (NECB)—be applied to the net rate of C accumulation in (or loss from [negative sign]) ecosystems. Net ecosystem carbon balance differs from NEP when C fluxes other than C fixation and respiration occur, or when inorganic C enters or leaves in dissolved form. These fluxes include the leaching loss or lateral transfer of C from the ecosystem; the emission of volatile organic C, methane, and carbon monoxide; and the release of soot and CO2 from fire. Carbon fluxes in addition to NEP are particularly important determinants of NECB over long time scales. However, even over short time scales, they are important in ecosystems such as streams, estuaries, wetlands, and cities. Recent technological advances have led to a diversity of approaches to the measurement of C fluxes at different temporal and spatial scales. These approaches frequently capture different components of NEP or NECB and can therefore be compared across scales only by carefully specifying the fluxes included in the measurements. By explicitly identifying the fluxes that comprise NECB and other components of the C cycle, such as net ecosystem exchange (NEE) and net biome production (NBP), we can provide a less ambiguous framework for understanding and communicating recent changes in the global C cycle.

Carbon in catchments: connecting terrestrial carbon losses with aquatic metabolism

For a majority of aquatic ecosystems, respiration ( R ) exceeds autochthonous gross primary production (GPP). These systems have negative net ecosystem production ((NEP) = (GPP) - R ) and ratios of (GPP)/ R of <1. This net heterotrophy can be sustained only if aquatic respiration is subsidized by organic inputs from the catchment. Such subsidies imply that organic materials that escaped decomposition in the terrestrial environment must become susceptible to decomposition in the linked aquatic environment. Using a moderate-sized catchment in North America, the Hudson River (catchment area 33 500 km 2 ), evidence is presented for the magnitude of net heterotrophy. All approaches (CO 2 gas flux; O 2 gas flux; budget and gradient of dissolved organic C; and the summed components of primary production and respiration within the ecosystem) indicate that system respiration exceeds gross primary production by ~200 g C m -2 year -1 . Highly 14 C-depleted C of ancient terrestrial origin (1000-5000 years old) may be an important source of labile organic matter to this riverine system and support this excess respiration. The mechanisms by which organic matter is preserved for centuries to millennia in terrestrial soils and decomposed in a matter of weeks in a river connect modern riverine metabolism to historical terrestrial conditions.

Whole-lake carbon-13 additions reveal terrestrial support of aquatic food webs

Ecosystems are supported by organic carbon from two distinct sources. Endogenous carbon is produced by photosynthesis within an ecosystem by autotrophic organisms. Exogenous carbon is produced elsewhere and transported into ecosystems. Consumers may use exogenous carbon with consequent influences on population dynamics, predator–prey relationships and ecosystem processes. For example, exogenous inputs provide resources that may enhance consumer abundance beyond levels supported by within-system primary production. Exogenous fluxes of organic carbon to ecosystems are often large, but this material is recalcitrant and difficult to assimilate, in contrast to endogenously produced organic matter, which is used more easily. Here we show, by the experimental manipulation of dissolved inorganic 13C in two lakes, that internal primary production is insufficient to support the food webs of these ecosystems. Additions of NaH13CO3 enriched the 13C content of dissolved inorganic carbon, particulate organic carbon, zooplankton and fish. Dynamics of 13C indicate that 40–55% of particulate organic carbon and 22–50% of zooplankton carbon are derived from terrestrial sources, showing that there is significant subsidy of these ecosystems by organic carbon produced outside their boundaries.

Early Warnings of Regime Shifts: A Whole-Ecosystem Experiment

High-frequency monitoring of manipulated and reference lakes enabled early detection of subsequent catastrophic regime shift. Catastrophic ecological regime shifts may be announced in advance by statistical early warning signals such as slowing return rates from perturbation and rising variance. The theoretical background for these indicators is rich, but real-world tests are rare, especially for whole ecosystems. We tested the hypothesis that these statistics would be early warning signals for an experimentally induced regime shift in an aquatic food web. We gradually added top predators to a lake over 3 years to destabilize its food web. An adjacent lake was monitored simultaneously as a reference ecosystem. Warning signals of a regime shift were evident in the manipulated lake during reorganization of the food web more than a year before the food web transition was complete, corroborating theory for leading indicators of ecological regime shifts.

Transformation of Freshwater Ecosystems by Bivalves

B ivalves (clams and mussels) are among the most familiar of aquatic organisms. Many have been used by humans for centuries as important sources of food and ornament, and some species are economically important pests, fouling water intakes and other structures. It is only recently, however, that ecologists have begun to understand that bivalves also play many important roles in ecosystems (e.g., Dame 1996). The functional importance of bivalves, especially in fresh water, is still not fully appreciated. For example, recent fresh water ecology I textbooks (Wetzel 1983, Horne and Goldman 1994, Allan 1995, Petts and Calow 1996) scarcely mention the ecological roles of bivalves (the words “bivalve, ” “clam,” and “mussel” do not even appear in the index of any of these books). By contrast,

TROPHIC CASCADES, NUTRIENTS, AND LAKE PRODUCTIVITY: WHOLE‐LAKE EXPERIMENTS

Responses of zooplankton, pelagic primary producers, planktonic bacteria, and CO2 exchange with the atmosphere were measured in four lakes with contrasting food webs under a range of nutrient enrichments during a seven-year period. Prior to enrichment, food webs were manipulated to create contrasts between piscivore dominance and planktivore dominance. Nutrient enrichments of inorganic nitrogen and phosphorus exhibited ratios of N:P > 17:1, by atoms, to maintain P limitation. An unmanipulated reference lake, Paul Lake, revealed baseline variability but showed no trends that could confound the interpretation of changes in the nearby manipulated lakes. Herbivorous zooplankton of West Long Lake (piscivorous fishes) were large-bodied Daphnia spp., in contrast to the small-bodied grazers that predominated in Peter Lake (planktivorous fishes). At comparable levels of nutrient enrichment, Peter Lakes areal chlorophyll and areal primary production rates exceeded those of West Long Lake by factors of approximatel...

Gas Exchange in Rivers and Estuaries: Choosing a Gas Transfer Velocity

We are writing this comment to call attention to large uncertainties in the estimates of CO2 flux from rivers and estuaries, a topic that has been receiving considerable attention recently (Raymond et al. 1997, 2000; Cai and Wang 1998; Frankignoulle et al. 1998). It is our view that there are too few direct measurements of the physical component of gas exchange (e.g., the gas transfer velocity, piston velocity, gas exchange coefficient, or k) for rivers and estuaries. While studies in streams, lakes, and marine systems have progressed to the point where the gas transfer velocity can be partially predicted from physical forcing functions (O’Connor and Dobbins 1958; Cole and Caraco 1998; Wanninkhof and McGillis 1999), this is not yet the case for estuaries and rivers. A comparison of gas transfer velocity measurements in estuaries and rivers reveals a general lack of agreement among studies and physically-based predictive models. Until we have a better understanding of the magnitude and causes of variation in estuarine gas transfer velocity estimates, it will be difficult to use gas exchange in rivers or estuaries to accurately mass-balance gases of interest. The exchange of CO2 between an aquatic ecosystem and the overlying atmosphere is an area of intense interest for several reasons: aquatic systems can be significant CO2 sources or sinks on a global or regional scale (Kling et al. 1991; Quay et al. 1992; Sarmiento and Sundquist 1992; Cole et al. 1994); the magnitude and direction of an ecosystems CO2 flux can provide important clues about metabolism in a given system (Depetris and Kempe 1993; Gattuso et al. 1993; Hamilton et al. 1995;

ZEBRA MUSSEL INVASION IN A LARGE, TURBID RIVER: PHYTOPLANKTON RESPONSE TO INCREASED GRAZING

Changes in the biomass of benthic bivalves can cause dramatic changes in total grazing pressure in aquatic systems, but few studies document ecosystem-level impacts of these changes. This study documents a massive decline in phytoplankton biomass con- current with the invasion of an exotic benthic bivalve, the zebra mussel ( Dreissena poly- morpha), and demonstrates that the zebra mussel actually caused this decline. In the fall of 1992 the zebra mussel became established at high biomass in the Hudson River Estuary, and biomass of mussels remained high during 1993 and 1994. During these 2 yr, grazing pressure on phytoplankton was over 10-fold greater than it had been prior to the zebra mussel invasion. This increased grazing was associated with an 85% decline in phyto- plankton biomass. Between 1987 and 1991 (pre-invasion), summertime chlorophyll aver- aged 30 mg/m 3 ; during 1993 and 1994 summertime concentrations were ,5 mg/m 3 . Over this same period, light availability increased, phosphate concentrations doubled, some planktonic grazers declined, and average flow was not different from the pre-invasion period. Thus, changes in these other factors were not responsible for phytoplankton declines. We developed a mechanistic model that reproduces the spatial and temporal dynamics of phytoplankton prior to the invasion of the zebra mussel (1987-1991). The model ac- curately predicts extreme declines in phytoplankton biomass after the invasion (1993-1994). The model demonstrates that zebra mussel grazing was sufficient to cause the observed phytoplankton decline. The model also allows us to test which features make the Hudson River sensitive to the impact of benthic grazers. The model suggests that the fate of light- scattering inorganic particles (turbidity) is a key feature determining the impact of benthic grazers in aquatic systems.