Summary
1. To evaluate the effect of habitat patch heterogeneity on abundance and growth of macroinvertebrates in arctic lakes, macroinvertebrate abundance, individual biomass, and potential food ...resources were studied in three patch types in two arctic lakes on the Alaskan North Slope near the Toolik Lake Field Station. An experiment was conducted to determine which sediment patch type supported higher growth rates for Chironomus sp., a commonly occurring macroinvertebrate.
2. Potential organic matter (OM) resources were significantly higher in both rock and macrophyte patches than in open‐mud patches. Total macroinvertebrate densities in both lakes were highest in rock patches, intermediate in macrophytes and lowest in open‐mud. The open‐mud patches also had lower species richness compared with other patch types. Additionally, individual biomass for one clam species and two chironomid species was significantly greater in rock patches than in open‐mud.
3. In a laboratory experiment, Chironomus showed two to three times greater mass increase in sediments from macrophyte and rock patches than from open‐mud patches. Rock and macrophyte experimental sediments had at least 1.5 × the percentage OM as open‐mud sediments.
4. Chlorophyll a appeared to be the best predictor for invertebrate abundances across all patch types measured, whereas OM content appeared to be the variable most closely associated with Chironomus growth.
5. Our results combined with previous studies show that the relationships between macroinvertebrate community structure, individual growth, and habitat heterogeneity are complex, reflecting the interaction of multiple resources, and biotic interactions, such as the presence or absence of a selective vertebrate predator (lake trout, Salvelinus namaycush).
Organic matter processing controls the flow of carbon and nutrients through ecosystems. Heterotrophic metabolism within ponds is supported by both terrestrial leaf litter and autochthonous ...production. We investigated the potential for the priming of leaf litter decomposition in small ponds using microcosms. We incubated senescent tulip poplar (
Liriodendron tulipifera
) leaf discs in the dark for 130 days either in contact with eutrophic pond sediments or isolated from sediment contact. Leaves that had been in contact with the sediments were significantly less tough and lost more carbon mass following the incubation than leaves that were not in contact with the sediments, indicating that they were decomposing faster. We calculated a positive priming effect of the sediments of 42% and 77% based on the change in toughness and C mass loss, respectively. We further found that leaf discs that were in contact with the sediments had significantly less fungal biomass, measured as ergosterol mass, and less leaf-derived N in fungal biomass than the leaf discs isolated from the sediments. These results indicate that the presence of the more labile organic matter of the sediments alters the rate of organic matter mineralization and the cycling of nitrogen and carbon.
We explore the role of lakes in carbon cycling and global climate, examine the mechanisms influencing carbon pools and transformations in lakes, and discuss how the metabolism of carbon in the inland ...waters is likely to change in response to climate. Furthermore, we project changes as global climate change in the abundance and spatial distribution of lakes in the biosphere, and we revise the estimate for the global extent of carbon transformation in inland waters. This synthesis demonstrates that the global annual emissions of carbon dioxide from inland waters to the atmosphere are similar in magnitude to the carbon dioxide uptake by the oceans and that the global burial of organic carbon in inland water sediments exceeds organic carbon sequestration on the ocean floor. The role of inland waters in global carbon cycling and climate forcing may be changed by human activities, including construction of impoundments, which accumulate large amounts of carbon in sediments and emit large amounts of methane to the atmosphere. Methane emissions are also expected from lakes on melting permafrost. The synthesis presented here indicates that (1) inland waters constitute a significant component of the global carbon cycle, (2) their contribution to this cycle has significantly changed as a result of human activities, and (3) they will continue to change in response to future climate change causing decreased as well as increased abundance of lakes as well as increases in the number of aquatic impoundments.
We compared physicochemical properties and rates of phytoplankton and epipelic primary production in two shallow lakes in the Alaskan arctic on eight occasions over three years. The two ...morphometrically similar lakes lacked defined inlets and had a mean depth of 2.2 m. The lakes differed with respect to glacial influence. Lake GTH 112 was continuously turbid due to resuspension of glacial silt from the lake bed, while GTH 114 showed higher clarity as it was situated on coarser glacial drift. The two lakes contrasted sharply in euphotic zone nutrient concentrations. Soluble reactive phosphorus, NO^sub 3^^sup -^ -N and NH^sub 4^^sup +^ -N concentrations averaged 0.17, 2.5 and 12.6 μM, respectively, in GTH 112, but were generally at or below the detection limit of 0.05 μM in GTH 114. Reduced light limited the ability of phytoplankton in GTH 112 to use the increased nutrients, and volume-based rates of phytoplankton primary production were similar between lakes. High turbidity in GTH 112 decreased the average percentage of total lake volume and sediment surface within the euphotic zone to 63% and 47%, respectively, compared with values of 88% and 85%, respectively, for GTH 114. Consequently, the average whole lake (phytoplankton plus epipelic) primary production rate in GTH 112 (8.5 mmol m^sup -2^ d^sup -1^) was significantly lower than the mean rate (12.3 mmol m^sup -2^ d^sup -1^) for GTH 114. The increase in turbidity affected benthic and pelagic habitats proportionately, as epipelon accounted for about 25% of total whole lake primary production in both lakes.PUBLICATION ABSTRACT
Patterns of sediment organic matter deposition in lakes reflect the factors that affect the production of organic matter in the lake and watershed and the removal of organic matter from the ...sediments. We surveyed the percent sediment organic matter of 22 lakes in the Alaskan Arctic and the rate of organic matter loss with sediment age in 3 lakes in the same region. The variation in sediment organic matter among lakes was greater than the variation between shallow and deep locations within the same lake, which is consistent with landscape-scale control of variation in sediment organic matter. In shallow water sediments, percent sediment organic matter was positively correlated with the amount of light reaching the sediments and the concentration of dissolved oxygen in the overlying water, suggesting that differences in organic matter content reflect differences in benthic production. The percent organic matter of the sediments in deep water was correlated with the percent organic matter in the sediments from shallow water but not environmental variables. The results suggest that variation in sediment organic matter in this region may be influenced by variation in benthic organic matter production more than by the loss of organic matter via mineralization.
The importance of biotic interactions in structuring communities can depend on seasonal changes in abiotic context. The crayfish and benthic-feeding fish of the headwaters of the New River have been ...shown to be ecosystem engineers due to their ability to reduce the amount of sediment accumulation in cobble beds during the summer. However, the temperature-driven seasonal reduction in the activity level of these organisms may reduce their impacts during the colder months. I measured the impact of benthic-feeding fish and crayfish on sediment accrual in the headwaters of the New River during the winter using a field exclosure experiment. I found that during the winter there was no effect of benthic-feeding fish and crayfish on the amount of sediment accumulating in cobble beds in the headwaters of the New River. These findings suggest that seasonal changes in the effect of ecosystem engineers need to be quantified for a complete understanding of the effect of these organisms.PUBLICATION ABSTRACT
In the headwaters of the New River in western North Carolina, the dominant crayfish species changes with increasing stream size. One transition occurs between third- and fourth-order streams. ...Cambarus chasmodactylus is the dominant species in third-order tributaries. Orconectes cristavarius is the dominant species in the fourth-order South Fork of the New River. While adult C. chasmodactylus are present in the South Fork, the young-of-the-year (YOY) of this species are absent despite evidence of reproduction. In this study we evaluated the factors that may be responsible for the absence of C. chasmodactylus YOY from the South Fork. A field experiment was used to evaluate the role of abiotic factors and competition with YOY O. cristavarius. There was no significant effect of either of these factors on mortality or growth of C. chasmodactylus YOY. The growth rate of O. cristavarius was 3× faster than that of C. chasmodactylus in this experiment. Since neither abiotic factors nor competition appeared responsible for the exclusion of C. chasmodactylus YOY from the South Fork we evaluated the potential importance of selective predation by rock bass (Ambloplites rupestris), a fish species that is common in the South Fork but virtually absent in the tributaries. In a laboratory experiment, C. chasmodactylus YOY experienced significantly higher mortality than O. cristavarius YOY in the presence of rock bass. Field observations and a laboratory experiment suggest that the two crayfish species differ in their anti-predator behaviors. Cambarus chasmodactylus was less likely to swim when initially disturbed and swam shorter distances than O. cristavarius. The differences in escape behavior and growth rate may contribute to the differences in the vulnerability of the two species to rock bass predation. Our results suggest that intense predation pressure exerted by the rock bass may contribute to the virtual exclusion of C. chasmodactylus YOY from the fourth-order South Fork.PUBLICATION ABSTRACT
The burial of organic matter within lake sediments can be a significant component of landscape carbon cycling. Whether organic matter deposited in lake sediments is sequestered or mineralized depends ...on factors limiting the decomposition rate of organic matter, such as temperature and the availability of oxygen. In stratified lakes, the distribution of temperature and oxygen is determined by the depth of the thermocline, and therefore sediment organic matter burial should be sensitive to changes in thermocline depth. Using a survey of more than 30 lakes over 3 years in the Alaskan Arctic, we found that thermocline depth during the summer was positively correlated with water transparency. Furthermore, using sediment incubations from 3 lakes, we found that variation in sediment oxygen demand is primarily affected by variation in temperature and the availability of oxygen with limited effect of the source of the sediments. Because variation in temperature and oxygen concentration in stratified lakes is mainly determined by the depth of thermocline, these results indicate that changes in transparency can have indirect effects on the rate of organic matter mineralization in lakes. A reduction in thermocline depth that results from decreased lake transparency may decrease the breakdown of sediment organic matter and increase the storage of organic carbon in lake sediments.
The movement of sediment between the lake bottom and water column of shallow lakes can be sizeable due to the large potential for resuspension in these systems. Resuspended sediments have been shown ...to alter phytoplankton community composition and elevate water column production and nutrient concentrations. We measured the summer sedimentation rates of two lakes in 2003 and six lakes in 2004. All lakes were shallow and located in the Alaskan Arctic. In 2004, turbidity, light attenuation, total sediment:chlorophyll
a
mass in the sediment traps, and thermal stratification were also measured in each of the lakes. The sediment:chlorophyll
a
mass was much greater than if the sediment was derived from phytoplankton production in all of the lakes, indicating that the source of the sedimenting material was resuspension and allochthonous inputs. Consistent with these findings, the temporal variation in sedimentation rate was synchronous between most lakes, and sedimentation rate was positively related to wind speed and rainfall suggesting that sedimentation rate was strongly influenced by landscape-scale factors (e.g., wind and rain events). Two of the lakes are located on deposits of loess that accumulated during past glacial periods. These two lakes had sedimentation rates that were significantly greater and more variable than any of the other lakes in the study, as well as high turbidity and light attenuation. Our results indicate that sedimentation in these shallow arctic lakes is supported primarily by allochthonous inputs and resuspension and that landscape-scale factors (e.g., weather and geology) impact on the transport of materials between the lake bottom and water column.
We examined the impact of small-scale commercial forestry on the structure and function of 6 headwater streams in the North Carolina Piedmont. During 2001-2003 terrestrial organic matter inputs, ...temperature, macroinvertebrate community composition and tolerance, leaf breakdown rate, and food web structure were quantified for 2 streams draining mature stands of managed loblolly pine, 2 streams draining mature hardwood forests, and 2 streams draining 3-year-old clear cuts, which had been replanted with loblolly pine. Streams in the clear-cuts and pine plantations were bordered by a 15 m hardwood buffer. Despite differences in watershed land-use, there were no significant differences in the organic matter supply or temperature between streams draining different forest types. However, algal biomass was significantly higher in clear-cut sites than forested sites, and was also higher in hardwood sites than pine sites. Streams draining the clear-cut sites contained lower macroinvertebrate richness and diversity, and fewer intolerant species, than streams draining pine and hardwood stands. Despite the differences in macroinvertebrates community composition, there was no difference among forest types in leaf-pack breakdown rates. Analysis of δ^sup 15^N and δ^sup 13^C natural abundance of functional feeding group indicated that the shredders and predators collected from streams draining clear-cuts had a δ^sup 15^N value that was enriched relative to the macroinvertebrates of streams draining pine and hardwood forests. This difference in δ^sup 15^N signature appears to be the result of the incorporation of riparian grass species in the clear-cuts, which have a higher δ^sup 15^N, into the diet of shredders. Pine sites had similar food webs to natural hardwood sites. Our results suggest that clear-cutting changes both the trophic dynamics and macroinvertebrate composition of low-order Piedmont streams in North Carolina despite the presence of hardwood buffers. However, large differences were not found between older pine and hardwood stands, indicating rapid recovery following re-growth of forest vegetation, when hardwood buffer strips were present.PUBLICATION ABSTRACT
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