Biogeochemistry of beetle-killed forests: explaining a weak nitrate response.

A current pine beetle infestation has caused extensive mortality of lodgepole pine (Pinus contorta) in forests of Colorado and Wyoming; it is part of an unprecedented multispecies beetle outbreak extending from Mexico to Canada. In United States and European watersheds, where atmospheric deposition of inorganic N is moderate to low (<10 kg⋅ha⋅y), disturbance of forests by timber harvest or violent storms causes an increase in stream nitrate concentration that typically is close to 400% of predisturbance concentrations. In contrast, no significant increase in streamwater nitrate concentrations has occurred following extensive tree mortality caused by the mountain pine beetle in Colorado. A model of nitrate release from Colorado watersheds calibrated with field data indicates that stimulation of nitrate uptake by vegetation components unaffected by beetles accounts for significant nitrate retention in beetle-infested watersheds. The combination of low atmospheric N deposition (<10 kg⋅ha⋅y), tree mortality spread over multiple years, and high compensatory capacity associated with undisturbed residual vegetation and soils explains the ability of these beetle-infested watersheds to retain nitrate despite catastrophic mortality of the dominant canopy tree species.

Phytoplankton growth regulation by dissolved P and mortality regulation by endogenous cell death over 35 years of P control in a Mountain Lake.

Dynamics of phytoplankton and phosphorus were quantified in Lake Dillon, Colorado, over 35 years of P control. The lake provides an example of early intervention for P enrichment rather than remediation of advanced eutrophication. Phosphorus control began with tertiary treatment of effluent, which caused a phytoplankton decline (8.1-4.5 µg L-1 chla); a second decline (4.6-2.5 µg L-1 chla) occurred later following replacement of failing septic systems. Results showed that bioavailable phosphorus (BAP) loading was the only significant correlate of phytoplankton biomass; total P loading was not significantly related to biomass measured as chlorophyll. Phytoplankton composition changed greatly over the study interval, even though there was no long-term trend in potential causes of phytoplankton abundance other than reduction in BAP. Gradual decline of BAP loading also appears to have been the cause of large, gradual changes in phytoplankton community composition. Factors typically assumed to control phytoplankton mortality accounted for only ~50% of phytoplankton biomass turnover; the balance of mortality appears to be accounted for by endogenous cell mortality.

Rationale for control of anthropogenic nitrogen and phosphorus to reduce eutrophication of inland waters.

Concentrations of phosphorus and nitrogen in surface waters are being regulated in the United States and European Union. Human activity has raised the concentrations of these nutrients, leading to eutrophication of inland waters, which causes nuisance growth of algae and other aquatic plants. Control of phosphorus often has had the highest priority because of its presumed leading role in limiting development of aquatic plant biomass. Experimental evidence shows, however, that nitrogen is equally likely to limit growth of algae and aquatic plants in inland waters, and that additions of both nutrients cause substantially more algal growth than either added alone. A dual control strategy for N and P will reduce transport of anthropogenic nitrogen through drainage networks to aquatic ecosystems that may be nitrogen limited. Control of total phosphorus in effluents is feasible and is increasingly being required by regulations. The control strategy for nitrogen in effluents is more difficult, but could be made more feasible by recognition that a substantial portion of dissolved organic nitrogen is not bioavailable; regulation should focus on bioavailable N (nitrate, ammonium, and some dissolved organic nitrogen) rather than total N. Regulation of both N and P also is essential for nonpoint sources.

Land use change and nitrogen enrichment of a Rocky Mountain watershed.

Headwater ecosystems may have a limited threshold for retaining and removing nutrients delivered by certain types of land use. Nitrogen enrichment was studied in a Rocky Mountain watershed undergoing rapid expansion of population and residential development. Study sites were located along a 30-km transect from the headwaters of the Blue River to Lake Dillon, a major source of drinking water for Denver, Colorado. Ground water in residential areas with septic systems showed high concentrations of nitrate-N (4.96 +/- 1.22 mg/L, mean +/- SE), and approximately 40% of wells contained nitrate with delta15N values in the range of wastewater. Concentrations of dissolved inorganic nitrogen (DIN) in tributaries with residential development peaked during spring snowmelt as concentrations of DIN declined to below detection limits in undeveloped tributaries. Annual export of dissolved organic nitrogen (DON) was considerably lower in residential streams, suggesting a change in forms of N with development. The seasonal delta15N of algae in residential streams was intermediate between baseline values from undeveloped streams and stream algae grown on wastewater. Between 19% and 23% of the annual N export from developed tributaries was derived from septic systems, as estimated from the delta15N of algae. This range was similar to the amount of N export above background determined independently from mass-balance estimates. From a watershed perspective, total loading of N to the Blue River catchment from septic and municipal wastewater (2 kg x ha(-1) x yr(-1)) is currently less than the amount from background atmospheric sources (3 kg x ha(-1) x yr(-1)). Nonetheless, nitrate-N concentrations exceeded limits for safe drinking water in some groundwater wells (10 mg/L), residential streams showed elevated seasonal patterns of nitrate-N concentration and ratios of DIN to total dissolved phosphorus, and seasonal minimum concentrations of nitrate-N in Lake Dillon have increased exponentially to 80 microg/L over the last decade from an initial value near zero. Results suggest that isotopic ratios in autotrophs can be used to detect and quantify increases in N enrichment associated with land use change. The biotic capacity of headwater ecosystems to assimilate increases in inorganic N from residential development may be insufficient to prevent nitrogen enrichment over considerable distances and multiple aquatic ecosystems downstream.

Cost and speed of locomotion for rotifers.

The hypothesis that the ciliary locomotion of rotifers is size limited and that it accounts of a significant portion of the energy budget was investigated using the genera Brachionus and Asplanchna. Speed of movement was measured among clones of different size in Brachionus, which shows little size variation through development. The same tests were done among individuals of different size within a clone of Asplanchna, which shows significant postembryonic size increase. In both cases, relative speed (body lengths per second) decreased significantly as body size increased. On this basis, and ecologically limiting size for ciliary locomotion is proposed. The actual cost of locomotion was measured for Brachionus; it is 62% of total metabolism, even though the theoretical (calculated) power requirements are well below 1% of total metabolism. Ciliary locomotion in the Rotifera thus appears to be extremely inefficient (low ratio of theoretical to actual power requirements). This hypothesis is supported indirectly by the sensitivity of speed to total metabolic rate in Brachionus: both plateau over the temperature range 20-32°C and decline in parallel outside this range. Unexpectedly high actual cost of locomotion is proposed as an important disadvantage of the Rotifera, partly offsetting the advantages accruing to them from small body size.

Metabolic responses to temperature change in a tropical freshwater copepod (Mesocyclops brasilianus) and their adaptive significance.

Metabolic rates of Mesocyclops brasilianus from Lake Valencia, Venezuela, were determined at several temperatures spanning the environmental range (22-28° C). The QO2's (oxygen consumption per unit weight) of all Mesocyclops stages from Lake Valencia are higher than most but not all QO2's from temperate copepod species that have been studied. The QO2 is essentially static through naupliar development and shows a sudden jump between N6 and CI, which probably results from the major change in morphology and behavior at this point in the life history. QO2 declines steadily between CI and adult stages. Acclimated copepodite and adult Mesocyclops show a decreasing metabolic rate with increasing temperature (i.e. Q10< 1.0) over the temperature range 26-28° C. This is the range of temperatures normally encountered during the daily vertical migration when the lake is thermally stratified (April-November). Since vertical migration would result in a compromise between a fully acclimated and an acute response, a nearly constant metabolic rate or a slight decline in metabolic rate in the warmer water would be expected in field populations. The results thus show that the metabolic rate of Mesocyclops is not reduced when it moves into deeper (cooler) water, as would be predicted by certain energy-based hypotheses that have been used to explain vertical migration. In contrast to the low Q10's between 26 and 28° C, copepodites and adults have very high Q10 values in the range 22-26° C. This indicates an adaptive decrease in metabolic rate which is thermally programmed to coincide with the cooler temperatures that are encountered during the mixing season (December-March), when a drastic change in ecological conditions occurs in the lake.Nauplii show evidence of the same seasonal response but without the superimposed plateau at high temperatures, which they would not need because they are weak migrators. Nauplii show a plateau at the lowest temperatures, however, which suggests that a fixed metabolic reduction occurs at the onset of mixing and metabolism is not altered thereafter with declining temperature.The change in QO2 with temperature generally supports the hypothesis that all Mesocyclops stages are adapted to hold a high, constant metabolic rate through the diel cycle but experience a seasonal reduction in metabolic rate in response to major ecological changes in the lake at the time of seasonal mixing.

Regulation and stability in fish assemblages of neotropical floodplain lakes.

Neotropical floodplain lakes provide an excellent opportunity to examine the regulation and stability of fish assemblages. At low water, when lakes are separated, fish are concentrated in the lakes and are presumably subject to strong interspecific interactions that can shape assemblage structure. At high water, when the lakes and river channels become broadly interconnected, ample potential exists for alteration of assemblage structure because eggs, larvae, and older fish may undergo spatial reshuffling among waterbodies. Twenty lakes of the Orinoco River floodplain in Venezuela were surveyed in the early and late dry seasons of two consecutive years. Marked differences in assemblage structure among lakes were established in the early dry season. Changes in assemblage structure during the dry season were large and detectable even at the ordinal level; they reflected a strong reduction of visually oriented fish relative to fish with adaptations to low light. Changes were similar in the two years and were apparently due to species-specific differences in mortality, which was high over the dry season. The annual flood is a strong natural fluctuation that greatly modifies assemblage properties during the wet season. Nevertheless, the potential for alteration of assemblage structure by reshuffling was not realized: assemblage properties early and late in the dry season were similar in the two years, indicating a regular and predictable annual cycle of change in assemblage structure. This regularity can be explained by mechanisms of regulation which appear to be linked to piscivory and the optical environment. In contrast with the prevailing views on neotropical fish assemblages, there seems to be a strong deterministic component to assemblage structure and dynamics in Orinoco floodplain lakes.

Ecological energetics of Chaoborus in a tropical lake.

Ecological energetics of Chaoborus brasiliensis from Lake Valencia, Venezuela, were studied between February 1979 and February 1980. Direct measurements were made of the respiration rate, assimilation efficiency, and growth rate of all 4 larval instars and of the pupae. For the larval stages, respiration increased as the 0.67 power of body mass. Respiration rates of the larvae, when corrected for body size and temperature, were extraordinarily low by comparison with the rates for most aquatic insects. The respiration rates of pupae were 3 times as high as those of larvae the same size. Assimilation rates increased significantly with body size for the larvae and differed slightly but significantly among food types. Assimilation efficiencies fell within the expected range for carnivores. The growth efficiencies were exceptionally high for instars II-IV by comparison with other small aquatic organisms. High growth efficiency for Chaoborus brasiliensis, and possibly for Chaoborus generally, is explained by a very low maintenance cost and may be a significant explanation for the wide distribution and high degree of ecological success in this primary carnivore of plankton communities.

Nonvisual feeding in a visual planktivore, Xenomelaniris venezuelae.

Studies of the diel feeding patterns of the planktivorous fish, Xenomelaniris venezuelae, in Lake Valencia, Venezuela, revealed that, although the fish is primarily a diurnal feeder, it consumes substantial numbers of Chaoborus larvae and pupae at night. A number of fish species are known which feed on plankton at night, but these fish are filter feeders and their diets largely consist of relatively small, nonevasive prey. Chaoborus, however, is large and agile. Predation by Xenomelaniris in the dark was also studied experimentally. Captured fish were placed in completely darkened aquaria with zooplankton from Lake Valencia. After several hours the plankton was removed and examined for evidence of feeding. The fish were found to consume Chaoborus pupae and fourth instar larvae but not other types of prey. The mode of feeding by Xenomelaniris in the dark is unknown.

RELATIONSHIPS BETWEEN GENETIC VARIABILITY AND LIFE-HISTORY FEATURES OF BONY FISHES.

Correlations between genetic variation and life-history variables were obtained for 80 species of bony fishes as a means of testing the hypothesis that genetic variation is directly related to 1) opportunity for balancing selection, as indicated by fecundity, and 2) environmental variation, as indicated by capacity for population increase. Genetic data were taken from the literature, and data on longevity, age at maturity, egg size, body size, and lifetime fecundity were taken from the literature where available and were otherwise estimated from other variables. Average heterozygosity does not increase significantly with increasing fecundity. However, heterozygosity is significantly associated with short generation times, quick maturation, small maximum size, and small eggs. Thus, heterozygosity appears to increase on a demographic continuum toward maximum values in species that are most strongly selected for maximizing the intrinsic rate of increase. Such species are associated with less stable environments. Thus, the results indicate a predominate role for environmental variation in controlling genetic variation of bony fishes.