Sally MacIntyre

Rapid and highly variable warming of lake surface waters around the globe

In this first worldwide synthesis of in situ and satellite-derived lake data, we find that lake summer surface water temperatures rose rapidly (global mean = 0.34°C decade−1) between 1985 and 2009. Our analyses show that surface water warming rates are dependent on combinations of climate and local characteristics, rather than just lake location, leading to the counterintuitive result that regional consistency in lake warming is the exception, rather than the rule. The most rapidly warming lakes are widely geographically distributed, and their warming is associated with interactions among different climatic factors—from seasonally ice-covered lakes in areas where temperature and solar radiation are increasing while cloud cover is diminishing (0.72°C decade−1) to ice-free lakes experiencing increases in air temperature and solar radiation (0.53°C decade−1). The pervasive and rapid warming observed here signals the urgent need to incorporate climate impacts into vulnerability assessments and adaptation efforts for lakes.

CO2 exchange between air and water in an Arctic Alaskan and midlatitude Swiss lake: Importance of convective mixing

[1] CO2 exchange between lake water and the atmosphere was investigated at Toolik Lake (Alaska) and Soppensee (Switzerland) employing the eddy covariance (EC) method. The results obtained from three field campaigns at the two sites indicate the importance of convection in the lake in driving gas flux across the water-air interface. Measurements were performed during short (1-3 day) periods with observed diurnal changes between stratified and convective conditions in the lakes. Over Toolik Lake the EC net CO2 efflux was 114 +/- 33 mg C m(-2) d(-1), which compares well with the 131 +/- 2 mg C m(-2) d(-1) estimated by a boundary layer model (BLM) and the 153 +/- 3 mg C m(-2) d(-1) obtained with a surface renewal model (SRM). Floating chamber measurements, however, indicated a net efflux of 365 +/- 61 mg C m(-2) d(-1), which is more than double the EC fluxes measured at the corresponding times (150 +/- 78 mg C m(-2) d(-1)). The differences between continous (EC, SRM, and BLM) and episodic (chamber) flux determination indicate that the chamber measurements might be biased depending on the chosen sampling interval. Significantly smaller fluxes (p < 0.06) were found during stratified periods (51 +/- 42 mg C m(-2) d(-1)) than were found during convective periods (150 +/- 45 mg C m(-2) d(-1)) by the EC method, but not by the BLM. However, the congruence between average values obtained by the models and EC supports the use of both methods, but EC measurements and the SRM provide more insight into the physical-biological processes affecting gas flux. Over Soppensee, the daily net efflux from the lake was 289 +/- 153 mg C m(-2) d(-1) during the measuring period. Flux differences were significant (p < 0.002) between stratified periods (240 +/- 82 mg C m(-2) d(-1)) and periods with penetrative convection (1117 +/- 236 mg C m(-2) d(-1)) but insignificant if convection in the lake was weak and nonpenetrative. Our data indicate the importance of periods of heat loss and convective mixing to the process of gas exchange across the water surface, and calculations of gas transfer velocity using the surface renewal model support our observations. Future studies should employ the EC method in order to obtain essential data for process-scale investigations. Measurements should be extended to cover the full season from thaw to freeze, thereby integrating data over stratified and convective periods. Thus the statistical confidence in the seasonal budgets of CO2 and other trace gases that are exchanged across lake surfaces could be increased considerably.

Buoyancy flux, turbulence, and the gas transfer coefficient in a stratified lake.

Gas fluxes from lakes and other stratified water bodies, computed using conservative values of the gas transfer coefficient k600, have been shown to be a significant component of the carbon cycle. We present a mechanistic analysis of the dominant physical processes modifying k600 in a stratified lake and resulting new models of k600 whose use will enable improved computation of carbon fluxes. Using eddy covariance results, we demonstrate that i) higher values of k600 occur during low to moderate winds with surface cooling than with surface heating; ii) under overnight low wind conditions k600 depends on buoyancy flux β rather than wind speed; iii) the meteorological conditions at the time of measurement and the inertia within the lake determine k600; and iv) eddy covariance estimates of k600 compare well with predictions of k600 using a surface renewal model based on wind speed and β.

Vertical and horizontal transport in lakes: linking littoral, benthic, and pelagic habitats

Benthic, pelagic and littoral habitats are linked by physical processes in lakes. Exchanges between littoral and pelagic regions or between sheltered embayments and open waters can occur when horizontal density differences are generated. The resulting flows have been observed in tropical, subtropical, and temperate lakes as a result of differences in rates of heating due to morphometry or variations in algal abundance and differences in the depths of wind mixing. Laboratory experiments show that such circulations can also occur under beds of floating vegetation. These circulations may be significant for the movement of nutrients within lakes. Whereas mixing induced by wind is often invoked as a mechanism for vertical transport, the depth of penetration of wind-induced mixing may be suppressed if rates of heating are high when wind speeds are high. Consequently, if nocturnal heat losses are high, the convective motions induced by thermal instabilities may be more important than wind mixing for ventilation of deep water with its typically higher concentrations of nutrients and dissolved gases. Even when mixing in shallow waters is induced by wind and by shear instabilities in the diurnal thermocline, mixing may be incomplete when stratification induced by diurnal heating is strong; gradients in nutrient concentrations and in phytoplankton distributions may persist. Resuspension of living and non-living particles from the benthos can reseed phytoplankton to the upper layers and alter rates of nutrient supply or scavenging of pollutants. Resuspension tends to be higher in shallow waters; subsequent horizontal transports convey these materials to the pelagic zone. Assessing the time and space scales for physical, chemical and biological processes facilitates the design of studies to show the coupling of these processes.

Evidence for sustained residence of macrocrustacean fecal pellets in surface waters off Southern California

Abstract Large fecal pellets produced by macrocrustaceans, probably euphausiids, were observed frequently in the upper 20 m of the Santa Barbara and Santa Cruz basins off southern California at abundances ranging from 500 to 98,000 pellets m −3 . Although sinking rates of these pellets, ranging from 18 to 170 m day −1 , were rapid enough to remove the pellets from surface waters within hours, up to 40% of the pellets in the field had peritrophic membranes which were partially or totally decayed. Laboratory aging studies indicate that these decomposing pellets were from 4 to 10 days old. Moreover, the daily calculated flux of pellets from surface water, based on sinking rate measurements, was many times the photosynthetic carbon production of the euphotic zone, suggesting that sustained daily production of pellets at the abundances we observed would not be possible. We hypothesize that a proportion of fecal pellets produced by vertically migrating macrocrustaceans at night do not sink immediately, but accumulate in surface waters. Turbulent mixing processes, which increase the residence time of fecal pellets in the mixed layer, partially explain the observed accumulation.

Nutrient fluxes from upwelling and enhanced turbulence at the top of the pycnocline in Mono Lake, California

Time series measurements of temperature at 15 depths and profiles of temperature-gradient microstructure were obtained during a period with strong wind forcing and subsequent calm in Mono Lake, California. The wind forcing increased the amplitude of basin-scale internal waves and energy at all wave frequencies relative to the calm period. Rates of dissipation of turbulent kinetic energy, ∈, were high (∈ > 10−6 m2 s−3) at the top of the pycnocline at both an inshore and an offshore site on a day when winds reached 10 m s−1 and on the following two days at an inshore site (∈ > 10−7 m2 s−3). The enhanced turbulence occurred at the depth of a subsurface temperature maximum (zTM) and coincidentally with elevated concentrations of NH4, reduced concentrations of chlorophyll a and particulate carbon, and increased abundance of the macrozooplankter Artemia monica. The NH4 at zTM was more dispersed and of lower concentration inshore than offshore and indicated greater turbulent transport inshore. Over the course of 4 days, chlorophyll a concentrations increased in the upper mixed layer, and C:N and C:Chl ratios decreased. Offshore, the change in C:N ratio indicated a relaxation of moderate nutrient deficiency. We hypothesize that excretion by A. monica and turbulent transport of the NH4 from the subsurface temperature maximum led to improved physiological status of phytoplankton in the upper mixed layer.

A global database of lake surface temperatures collected by in situ and satellite methods from 1985–2009

Global environmental change has influenced lake surface temperatures, a key driver of ecosystem structure and function. Recent studies have suggested significant warming of water temperatures in individual lakes across many different regions around the world. However, the spatial and temporal coherence associated with the magnitude of these trends remains unclear. Thus, a global data set of water temperature is required to understand and synthesize global, long-term trends in surface water temperatures of inland bodies of water. We assembled a database of summer lake surface temperatures for 291 lakes collected in situ and/or by satellites for the period 1985–2009. In addition, corresponding climatic drivers (air temperatures, solar radiation, and cloud cover) and geomorphometric characteristics (latitude, longitude, elevation, lake surface area, maximum depth, mean depth, and volume) that influence lake surface temperatures were compiled for each lake. This unique dataset offers an invaluable baseline perspective on global-scale lake thermal conditions as environmental change continues.

PHYSICAL STRUCTURE OF LAKES CONSTRAINS EPIDEMICS IN DAPHNIA POPULATIONS

Parasites are integral parts of most ecosystems, yet attention has only recently focused on how community structure and abiotic factors impact host-parasite interactions. In lakes, both factors are influenced by habitat morphology. To investigate the role of habitat structure in mediating parasitism in the plankton, we quantified timing and prevalence of a common microparasite (Metschnikowia bicuspidata) in its host, Daphnia dentifera, in 18 lakes that vary in basin size and shape. Over three years, we found substantial spatial and temporal variation in the severity of epidemics. Although infection rates reached as high as 50% in some lakes, they did not occur in most lakes in most years. Host density, often considered to be a key determinant of disease spread, did not explain a significant amount of variation in the occurrence of epidemics. Furthermore, host resistance does not fully explain this parasites distribution, since we easily infected hosts in the laboratory. Rather, basin shape predicted epidemics well; epidemics occurred only in lakes with steep-sided basins. In these lakes, the magnitude of epidemics varied with year. We suggest that biological (predation) and physical (turbulence) effects of basin shape interact with annual weather patterns to determine the regional distribution of this parasite.

Energy input is primary controller of methane bubbling in subarctic lakes

Emission of methane (CH4) from surface waters is often dominated by ebullition (bubbling), a transport mode with high-spatiotemporal variability. Based on new and extensive CH4 ebullition data, we ...

A new large volume bioluminescence bathyphotometer with defined turbulence excitation

Abstract A vertical profiler for measuring stimulated bioluminescence is described. Unique features include statistically rigorous sampling, facilitated by high pumping rates (up to 441 s −1 ), hydrodynamically calibrated excitation at the entrance to the large volume (11 1) detection chamber, and the capability of rapid vertical deployment to depths of 500 m at descent velocities up to 50 m min −1 , while collecting data at 64 kilobaud. Radiometric, hydrodynamic and biological calibrations of this High Intake. Defined Excitation Bathyphotometer (HIDEX-BP) are described. Data collected during Biowatt 1987 with the HIDEX-BP are presented.