Nina F. Caraco

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.

Major role of marine vegetation on the oceanic carbon cycle

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Major role of marine vegetation on the oceanic carbon cycle C. M. Duarte, J. J. Middelburg, N. Caraco

Human Impact on Erodable Phosphorus and Eutrophication: A Global Perspective

Human actions—mining phosphorus (P) and transporting it in fertilizers, animal feeds, agricultural crops, and other products—are altering the global P cycle, causing P to accumulate in some of the world’s soil. Increasing P levels in the soil elevate the potential P runoff to aquatic ecosystems (Fluck et al. 1992, NRC 1993, USEPA 1996). Using a global budget approach, we estimate the increase in net P storage in terrestrial and freshwater ecosystems to be at least 75% greater than preindustrial levels of storage. We calculated an agricultural mass balance (budget), which indicated that a large portion of this P accumulation occurs in agricultural soils. Separate P budgets of the agricultural areas of developing and developed countries show that the rate of P accumulation is decreasing in developed nations but increasing in developing nations.

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.

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,

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.

LINKING PLANKTONIC BIOMASS AND METABOLISM TO NET GAS FLUXES IN NORTHERN TEMPERATE LAKES

Plankton communities in oligotrophic waters are characteristically dominated by the biomass of heterotrophs, including bacteria, micro-, and macrozooplankton. It has been generally assumed that these inverted biomass pyramids are the direct result of high specific production rates of phytoplankton and a tight coupling between producers and consumers. There are, however, at least two alternative hypotheses: (1) heterotrophic biomass turnover is much slower in oligotrophic than eutrophic systems; and (2) oligotrophic planktonic communities are significantly subsidized by allochthonous organic matter. In this study we assessed these hypotheses by establishing the relationship between plankton biomass structure (partition between auto- and heterotrophs), plankton function (plankton primary production and respiration) and whole-lake gas (O2 and CO2) fluxes in 20 temperate lakes that span a large range in primary production. We show that the balance of phytoplankton production and community respiration (P/R rat...

Carbon dioxide concentration and atmospheric flux in the Hudson River

We made direct measurements of the partial pressure of CO2 (PCO2) in the tidal-freshwater portion of the Hudson River Estuary over a 3.5-yr period. At all times the Hudson was supersaturated in CO2 with respect to the atmosphere. PCO2 in surface water averaged 1125±403 (SD) μatm while the atmosphere averaged 416±68 μatm. Weekly samples at a single, mid-river station showed a pronounced and reproducible seasonal cycle with highest values (∼2000 μatm) in mid-to-late summer, and lowest values (∼500 μatm) generally in late winter. Samples taken along the length of the 190-km section of river showed a general decline in CO2 from north to south. This decline was most pronounced in summer and very slight in spring. Diel and vertical variation were small relative to the standing stock of CO2. Over six diel cycles, all taken during the algal growing season, the mean range was 300±114 μatm. CO2 tended to increase slightly with depth, but the gradient was small, about 0.5 μmol m−1, or an increase of 190 μatm from top to within 1 m of the bottom. For a large subset of the samples (n=452) we also calculated CO2 from measurements of pH and total DIC. Calculated and measured values of CO2 were in reasonably good agreement and a regression of calculated versus measured values had a slope of 0.85±0.04 and an r2 of 0.60. Combining our measurements with recent experimental studies of gas exchange in the Hudson, we estimate that the Hudson releases CO2 at a rate of 70–162 g C m−2 yr−1 from the river to the atmosphere.

Contrasting impacts of a native and alien macrophyte on dissolved oxygen in a large river

In aquatic systems low dissolved oxygen (DO) has been identified as a serious water quality problem. Here we use empirical data and modeling to explore the hypothesis that the introduction of an alien aquatic macrophyte (Trapa natans) may have had dramatic impacts on the frequency and extent of low DO events in the Hudson River. Continuous measurements with moored instruments demonstrated that in large macrophyte beds dom- inated by a native species (Vallisneria americana) DO never declined below 5 mg/L during the summer growing season. In contrast, during this same time period, extremely low DO was common in large beds dominated by Trapa natans, with DO values below 2.5 mg/L occurring up to 40% of the time. This difference in DO can be modeled based on species differences in the balance of respiration and in-water photosynthesis. The low DO values in Trapa beds suggest that these beds may be poor habitats for sensitive fish and inver- tebrates and that redox sensitive chemical reactions may be altered within Trapa beds.

Nitrogen Loading of Rivers as a Human-Driven Process

The ecology of areas populated by humans is viewed in a number of ways in this book. In this chapter, we examine the biogeochemistry of nitrogen in the major rivers of the world as a function of the number of humans inhabiting the watersheds of these rivers. This study should be of interest for several reasons. First, it is an ecological study that includes, quite explicitly, the humans within the system. Further, the coastal margins tend to have the greatest concentration of urban areas and human population density. For example, even in the United States, a country with a great deal of interior relative to coastal zone, 53% of residents live within 50 miles of the coast (Schubel and Bell 1991). The rivers that pass through these coastal zones are a major source of nutrients, including nitrogen, to coastal waters. Nitrogen (N) is an essential plant nutrient in aquatic systems and is considered a limiting factor for primary productivity in coastal marine systems (Caraco et al. 1987; Howarth 1988). Significant increases of riverine or estuarine N, therefore, can lead to undesirable consequences such as nuisance algal blooms, anoxia or hypoxia, or loss of native or economically important species (D’Elia 1987).