Robert G. Wetzel

Gradient-dominated ecosystems: sources and regulatory functions of dissolved organic matter in freshwater ecosystems

The emergent wetland and littoral components of the land-water zone are functionally coupled by the amounts and types of dissolved organic matter that are released, processed, transported to, and then further processed within the recipient waters. Operational couplings and integrations in freshwater ecosystems occur along physical and metabolic gradients of a number of scales from micrometer to kilometer dimensions. The operation and turnover of the microbial communities, largely associated with surfaces, generate the metabolic foundations for material fluxes along larger-scale gradients.

Habitat Partitioning and Competitive Displacement in Cattails (Typha): Experimental Field Studies

The aquatic plants Typha latifolia and T. angustifolia are observed to be strongly segregated along a gradient of increasing water depth with T. latifolia restricted to depths of less than 80 cm and T. angustifolia to depths greater than 15 cm. Transplantation of both species along the gradient in the absence of competitors showed that T. latifolia was little affected by the presence of T. angustifolia but T. angustifolia was capable of growing over the entire gradient. The loss of precompetitive distribution was not statistically significant for T. latifolia compared to a 39.6% loss for T. angustifolia. It was further observed that overlap was reduced by 43.5% during the course of the growing season. Rhizomes transplanted into natural stands failed to survive, further demonstrating that competition was actively operating to maintain zonation between species. The basis for habitat partitioning appears to be a difference in morphology whereby T. latifolia was prevented from growing in deep water because of the higher cost of producing broader leaves but better able to compete for light in shallow water because of its greater leaf surface area. Coexistence is largely the result of a deep-water refuge for the competitively inferior T. angustifolia although other factors may be involved.

Periphyton of Freshwater Ecosystems

The dynamics of periphyton communities are on a long term the result of maturation of the system or eutrophication. Yearly fluctuations are brought about by a number offactors, e.g. availability of nutrients, light, or substratum, by pelagic influence, or grazing. For a better understanding of these population dynamics, standard methods in the separation and culture of periphytic organisms are needed.

EFFECTS OF CLIMATE CHANGE ON FRESHWATER ECOSYSTEMS OF THE SOUTH‐EASTERN UNITED STATES AND THE GULF COAST OF MEXICO

The south-eastern United States and Gulf Coast of Mexico is physiographically diverse, although dominated by a broad coastal plain. Much of the region has a humid, warm temperate climate with little seasonality in precipitation but strong seasonality in runoff owing to high rates of summer evapotranspiration. The climate of southern Florida and eastern Mexico is subtropical with a distinct summer wet season and winter dry season. Regional climate models suggest that climate change resulting from a doubling of the pre-industrial levels of atmospheric CO 2 may increase annual air temperatures by 3-4°C. Changes in precipitation are highly uncertain, but the most probable scenario shows higher levels over all but the northern, interior portions of the region, with increases primarily occurring in summer and occurring as more intense or clustered storms. Despite the increases in precipitation, runoff is likely to decline over much of the region owing to increases in evapotranspiration exceeding increases in precipitation. Only in Florida and the Gulf Coast areas of the US and Mexico are precipitation increases likely to exceed evapotranspiration increases, producing an increase in runoff (...)

Decomposition of aquatic angiosperms. II. Particulate components

The decomposition of particulate organic matter (POM) from five species of freshwater vascular macrophytes was analyzed over a 180-day period under controlled conditions of temperature and oxygen (10 and 25°C; aerobic and anaerobic). In addition, a series of seasonal decomposition experiments were conducted in situ under different lake conditions. Plant species studied included the emergent Scirpus acutus Bigelow, the floating-leaved lily Nuphar variegatum Engelm., and the submersed Myriophyllum heterophyllum Michx., Najas flexilis (Willd.) Rostk. & Schmidt, and the submersed bulrush Scirpus subterminalis Torrey. At 2, 4, 10, 24, 48, 90, and 180-day intervals, organic weight, ash, carbon, nitrogen, total non-structural carbohydrates (TNC), and fiber components (lignin, cellulose, and hemicellulose) were determined under each of the experimental conditions. A model was developed that fit the observed weight-loss data, which demonstrated that decay-rate coefficients decreased exponentially with time in response to increasing resistance to decomposition of residual organic constituents. The fastest decomposing species, Nuphar, exhibited losses from 60 (cold, anaerobic) to 90% (warm, aerobic) over a 180-day period; the slowest, Scirpus acutus, lost only 20 and 40% under comparable conditions. TNC content decreased more rapidly under warm aerobic than under cold anaerobic conditions. Reduction of TNC was greatest in Nuphar, less in Myriophyllum and Najas, and lowest in the two Scirpus species. Structural carbohydrates decreased slowly among all species with the lignin fraction being the most resistant to degradation. However, decay rates of the macrophytes were negatively correlated to total fiber content rather than to individual fiber components (hemicellulose, cellulose, and lignin). Floating-leaved plants contained the least amount of structural tissue and decayed most rapidly, followed by submersed and then emergent species. Scirpus subterminalis, although submersed, retains the structural characteristics of the emergent species of this genus and decayed in much the same way as S. acutus. The C:N ratios of tissue of Scirpus acutus were higher than for other species and continuously declined during decomposition under warm, aerobic conditions. In this species, the C:N ratios decreased very slowly under cold, anaerobic conditions. The C:N ratios of the other species were constant after initial rapid declines. Among all species, the proportion of nitrogen in the dissolved organic and inorganic matter of the media decreased rapidly to low levels during decomposition at both low and high temperatures under anaerobic conditions; reductions were less when oxygen was available and decreased least in cultures of the two Scirpus species. Thus, resistance of particulate tissue to microbial decomposition was strongly influenced by the composition of the plant species. The floating-leaved plant decomposed faster than the submersed plants, which decomposed more rapidly than the emergent species. Decay rates were related to initial nitrogen and fiber contents, with high-nitrogen, low-fiber plants decomposing most rapidly. Unlike the leached dissolved organic matter (DOM) of the plants whose rate of decomposition was found to be mostly influenced by oxygen availability, the rate of conversion of particulate matter to carbon dioxide and/or DOM was regulated primarily by temperature, tissue nitrogen, and fiber content. All species showed similar trends in decomposition under both laboratory and in situ conditions, but the species-specific rates differed under the various laboratory conditions of temperature and oxygen.

Nutrient additions by waterfowl to lakes and reservoirs: predicting their effects on productivity and water quality

Lakes and reservoirs provide water for human needs and habitat for aquatic birds. Managers of such waters may ask whether nutrients added by waterfowl degrade water quality. For lakes and reservoirs where primary productivity is limited by phosphorus (P), we developed a procedure that integrates annual P loads from waterfowl and other external sources, applies a nutrient load-response model, and determines whether waterfowl that used the lake or reservoir degraded water quality. Annual P loading by waterfowl can be derived from a figure in this report, using the days per year that each kind spent on any lake or reservoir. In our example, over 6500 Canada geese (Branta canadensis) and 4200 ducks (mostly mallards, Anas platyrhynchos) added 4462 kg of carbon (C), 280 kg of nitrogen (N), and 88 kg of P y−1 to Wintergreen Lake in southwestern Michigan, mostly during their migration. These amounts were 69% of all C, 27% of all N, and 70% of all P that entered the lake from external sources. Loads from all external sources totaled 840 mg P m−2 y−1. Application of a nutrient load-response model to this concentration, the hydraulic load (0.25 m y−1), and the water residence time (9.7 y) of Wintergreen Lake yielded an average annual concentration of total P in the lake of 818 mg m−3 that classified the lake as hypertrophic. This trophic classification agreed with independent measures of primary productivity, chlorophyll-a, total P, total N, and Secchi disk transparency made in Wintergreen Lake. Our procedure showed that waterfowl caused low water quality in Wintergreen Lake.

Decomposition of aquatic angiosperms. III. Zostera marina L. and a conceptual model of decomposition

Abstract The decomposition of particulate and dissolved organic matter of a marine angiosperm, Zostera marina L., was followed over 180 days in the laboratory under three conditions of oxygen (aerobic, anaerobic, aerobic-to-anaerobic) at each of two temperatures (10 and 25°C). Samples of detrital material were taken after 2, 4, 10, 24, 48, 90, and 180-day intervals for analysis of organic weight loss, ash content, particulate carbon and nitrogen, total non-structural carbohydrates, hemicellulose, cellulose, lignin, and for UV absorbance, fluorescence activity, and total dissolved organic carbon (DOC) in the synthetic seawater media. The DOC analyses were performed on four molecular weight fractions separated by membrane ultrafiltration and on the whole fraction. In addition, the microbial biomass and activity associated with the detrital material was determined each sampling day by ATP content and dehydrogenase activity assays. Zostera tissue was found to be much more resistant to decomposition than were freshwater angiosperms examined concurrently under similar conditions. Significant weight loss over the study period occurred only under warm, aerobic conditions. Concentrations of fiber components of Zostera before decomposition were within the range of those of freshwater species studied. The concentrations of the fiber components (hemicellulose, cellulose, lignin) of the particulate detritus changed little. Likewise, particulate nitrogen was initially similar in value to that of freshwater plants, and changed in time depending on the physical conditions of decomposition, usually first increasing then declining. ATP content of the particulate detritus exhibited similar trends, namely increasing then declining values, over various time scales depending on decomposition conditions of temperature and oxygen availability. Production and utilization of dissolved organic matter (DOM) was greatest at the higher temperature, but more resistant DOM, as determined by increased UV absorbance and fluorescence activity of the material, persisted under these conditions. The increased resistance of Zostera marina to decomposition in comparison with several freshwater species cannot be explained by initial contents of nitrogen or fiber components, as was found for the latter plants. It is hypothesized that such resistance results from the ultrastructure of the tissue, and is due to adaptation of this species to a physically harsh environment. Based on data of this and previous studies, a conceptual model is described in which all forms of decomposition of naturally occurring plant detritus in aquatic ecosystems are proposed to advance, with respect to rates of decomposition, through three phases. Rates of decay are controlled primarily by temperature, dissolved oxygen, nutrient availability, particle size, and tissue resistance, and these parameters interact directly with each other to determine the myriad of decay rates observed under natural and laboratory conditions.

19 – Dissolved Organic Carbon: Detrital Energetics, Metabolic Regulators, and Drivers of Ecosystem Stability of Aquatic Ecosystems1

Publisher Summary Trophic structure has dominated evaluations of the rates of energy fixation by primary producers in terrestrial and aquatic pelagic communities, and the rate of transfer of efficiencies of this energy to higher trophic levels. Flows of energy within the trophic structures and their variants specifically addressed predation, that is, ingestion of particles of organic matter. Trophic structure and energy fluxes were evaluated predominantly in pelagic regions on the basis of ingestion of particulate organic matter by living organisms, and the effects of consumption on the population dynamics of trophic levels. Metabolism of particulate detritus (nonliving) especially dissolved organic matter from many pelagic and nonpelagic autochthonous and from allochthonous sources, dominates both material and energy fluxes. In providing an overview and syntheses of the evaluations of organic carbon, a general consensus has emerged along several directions of thought concerning the functions of dissolved organic matter in aquatic ecosystems. The inland water systems are being treated as integrated ecosystems with the increasing and full appreciation that the metabolism of the lakes and rivers is dependent largely or to a major extent on metabolism in the adjacent drainage basin. Commonality of function is found amid the plethora of individual habitat diversity. This important merger is especially evident in the recognition that the processes are functionally similar in freshwater and marine ecosystems. Differences are apparent at the sources of organic matter, which affect the rates of utilization, but not the operation of utilization. The rates of utilization differ because of the dominance of more recalcitrant organic substrates in inland water ecosystems.

Decomposition of aquatic angiosperms. I. Dissolved components

The decomposition of dissolved organic matter (DOM) released from freshwater vascular macrophytes was investigated over a 180-day period under controlled conditions of temperature and oxygen (10 and 25°C; anaerobic and aerobic). Plant species studied were the emergent bulrush Scirpus acutus Bigelow, the floating-leaved water lily Nuphar variegatum Engelm., two submerged plants, Myriophyllum heterophyllum Michx., Najas flexilis (Willd.) Rostk. & Schmidt, and the submersed bulrush, Scirpus subterminalis Torrey. The concentrations of dissolved organic carbon (DOC) released by autolysis and by bacterial metabolism were analyzed in total and fractionated into four molecular weight categories (< 30 000, < 10 000, < 1000, and < 500 Daltons). Ultraviolet absorptive and fluorescence characteristics of each fraction were also analyzed. Decomposition of DOM was most rapid under aerated conditions at either temperature and resulted in declining or consistently low concentrations of DOC in all molecular weight fractions. Fluorescence and UV absorbance of low molecular weight fractions were rapidly reduced to low values, indicative of rapid removal of small compounds with unsaturated bonds or ring structures by bacterial degradation, complexing, or absorption to particulate phases. While the amount of high molecular weight DOC decreased, spectral properties indicated an increasing dominance of complex humic-type compounds. Under aerobic conditions, small and large molecular weight fractions showed consistent losses in total organic carbon at both temperatures. The presence of oxygen was more effective in control of rates of conversion of DOM than was temperature over the range examined in these experiments. A temperature effect was observed, however. At the lower temperature, when anaerobic, DOM was only slowly metabolized, and even low molecular weight fractions accumulated carbon. At the higher temperature, DOM was eventually decomposed even under anaerobic conditions. Decomposition of DOM in aerated cultures proceeded slightly faster at 25 than at 10°C.

Photochemical transformation of allochthonous organic matter provides bioavailable nutrients in a humic lake

Carbon, nitrogen and phosphorus associated with dissolved organic matter (DOM) form a large potential source of nutrients and energy for bacterio- and phytoplankton. The role of solar radiation in the transformation of DOM into inorganic and bioavailable forms was investigated in a humic boreal Lake Valkea-Kotinen. The concentrations of nitrate+nitrite, inorganic phosphorus and inorganic carbon increased, but those of ammonium decreased in <0.2-μm filtered hypolimnetic water during 1-day exposures to solar radiation. In epilimnetic water, solar radiation increased the concentration of ammonium at a rate equivalent to the rate of atmospheric deposition of inorganic nitrogen. When indigenous bacteria of Lake Valkea-Kotinen were inoculated into sunlight-exposed waters, bacteria achieved higher biovolume and productivity and incorporated carbon, nitrogen and phosphorus at greater rates than those grown in non-exposed waters. Bacteria mineralized dissolved organic carbon 92-375 % more in exposed than in non-exposed waters. Thus, in addition to direct photochemical mineralization, solar radiation increased metabolic mineralization of organic carbon by bacteria. Solar radiation decreased the activity of phosphomonoesterase during exposures down to <1% of the initial values. However, after 4-d bioassay the activity of phosphomonoestrase in the exposed waters exceeded that in the non-exposed water. Results showed that solar radiation transformed dissolved organic nitrogen, phosphorus and carbon into forms readily available to phyto- and bacterioplankton. The photochemical supply of nutrients increased the production of bacterioplankton and can be expected also to increase production of phytoplankton.