Understanding the frequency and time scale of episodic events is a first step toward understanding their importance in lake trophic dynamics. Vertical and horizontal patterns can form, evolve, and decay during a single season. However, little is known about the mechanisms of transport across these interfaces in large lakes, the temporal progression of events, and the time course of water mass and trophic stimulation. The focus of this remote sensing effort is on one of the most crucial linkages to biotic systems, at the base of foodwebs, where limiting nutrients promote phytoplankton growth. Our general thesis is that remote sensing technology will facilitate timely and cost effective analyses of changes in particle and chlorophyll abundances seasonally and interannually. The time series imagery revealed significant productivity pulses (elevated Chl a) associated with resuspended sediments in Lake Michigan during fall, winter, and spring months and the development of a productive southern shore coastal corridor during spring in Lake Superior. Analysis of simultaneously collected lake surface temperature imagery from the AVHRR revealed distinct horizontal temperature gradients from nearshore to offshore in late March and April in Lake Michigan and during April through July in Lake Superior. In both systems, the thermal bar (4 degree C water of highest density) which was identified in AVHRR lake surface temperature imagery, separated nearshore from offshore waters. In southern Lake Michigan, the result is two distinct chlorophyll populations separated by the thermal bar. In Lake Superior, the thermal bar defines the nearshore coastal corridor where elevated chlorophyll and suspended sediments are found. These events were previously
In Lake Michigan, winter and early spring storms massively resuspend materials with characteristics of sediment depositional regions and these materials are transported offshore into the center of the lake. We have calculated that these events can resuspend over one million MT of particulate matter and sediment traps record the offshore passage of these materials. Several years of trap collection at offshore sites show a range in mass and nutrient fluxes that span a factor of ten. The implication is that large events are crucial in the movement of particulate matter and associated chemical constituents.
Light absorption by photosynthetic pigments is an important process for phytoplankton growth and primary production. Typically, phytoplankton absorption has been determined at the community level using bulk measurements. Use of microphotometric techniques enables the direct measurement of the in vivo absorption efficiency factor (lambda) of individual cells. This technique combines microscopic and spectrophotometric methods to provide a measure of spectral transmittance of individual particles, from which lambda can then be calculated. Applications of this method were used to study phytoplankton populations in southeastern Lake Michigan. Light is an important variable controlling the development of the spring diatom community in Lake Michigan. This study examined the impact of a recurrent coastal plume on variations in light availability and the associated absorption characteristics of three phytoplankton groups in the vicinity of the plume. lambda for a given group did not significantly vary with depth. However, temporal variations in the magnitude of lambda did occur within groups as reflected in changes in the blue:red ratios with time. The three groups that were studied - Aulacoseira islandica, Aulacoseira subarctica, and centric diatoms of 8-12 um diameter - differed in spectral shape of lambda in the blue portion of the spectrum. For a given time and species, comparisons of lambda across plume gradients showed no apparent differences in magnitude or spectral shape. These results will be used in conjunction with other single-cell measurements to examine spatial and temporal patterns in taxon-specific
The development of a "seed bank" of resting eggs depends upon yearly production, hatching schedules, survival during burial, and long-term viability. We show that cladoceran egg banks vary in genotype frequencies through time, showing that populations evolve relatively rapidly at specific sites. However, in large lakes, storm events can purge a portion of the buried eggs from the "seed bank", resurrecting both ancient genotypes (intraspecific component) and species (interspecific component). On the scale of decades to centuries, the frequency of episodic events could be important for maintaining genetic and community diversity. Examples from freshwater estuaries (Portage Lake) and large (Lake Michigan, Lake Superior) lakes are used to clarify aspects of the investigation methods.
A set of laboratory and shipboard experiments was conducted in southern Lake Michigan in 1998 and 1999 to examine the growth and grazing loss rates of microzooplankton during the winter-spring transition period. The study was a part of the Episodic Event Great Lakes Experiment (EEGLE). Despite low ambient temperatures (2 to 4 deg C, the average specific growth rate of microzooplankton (predominantly ciliates) was close to 0.4 d-1 and in certain ciliate taxa it exceeded 1 d-1. The predominant calanoid copepods, Diaptomus sicilis and Limnocalanus macrurus, selectively grazed the hymenostomatid ciliate Lembadion lucens and oligotrichs. Nauplii of Diaptomus also actively grazed ciliates. These preliminary results suggest that microzooplankton can be an important part of the diet of over-wintering planktonic copepods and their nauplii.
Previous observations of near-bottom conditions and sediment resuspension in Lake Michigan have concentrated on the areas of high sediment deposition in the southeastern part of the lake and on Indiana Shoals in the southwestern part. No previous observations have been made in the western areas of the lake adjacent to the Wisconsin bluffs, primarily because these waters were considered to be non-depositional and thus limited in the amount of resuspendable sediment available for transport. During 1998, the first EEGLE field year, we used an instrumented tripod to make measurements intended to test the hypothesis that the apparently recurrent winter-spring resuspension event (misnamed "plume") includes new material eroded from the Wisconsin near-shore. Almost 2300 hours of simultaneous near-bottom temperature, transparency and current velocity, along with surface wave height data were collected in three deployments done in April, August, and November. The results show that the local sediment, a sandy cohesive clay, is indeed resuspended in response to the combined action of currents and surface waves. Net horizontal sediment flux in April was almost directly parallel to the shore and towards the south. In November, however, the net horizontal sediment flux was onshore, perhaps representing the temporary storage of a more easily resuspendable pool of sediment.
A recurrent coastal turbidity plume (RCP) is a consistent feature observed in southern Lake Michigan during the spring isothermal period. The plume feature occurs at roughly the same time as the spring diatom bloom and its impact on phytoplankton growth is unclear. A goal of the Coastal Ocean Processes-Episodic Events in the Great Lakes Experiment (CoOP-EEGLE) is to determine the impact of transport of materials along the coastal margin on biological processes. Current hypotheses differ as to the primary mechanism by which the plume may affect phytoplankton growth. Although light availability is believed to be important in regulating growth, the RCP may also influence the availability of limiting nutrients, principally phosphorus. A prerequisite to resolving questions about the impact of the RCP on phytoplankton growth is a precise description of the optical conditions within the plume and the light utilization efficiencies of phytoplankton. Inherent optical properties of the RCP and adjacent waters were characterized during March 1999 in southeastern Lake Michigan using a combination of in situ instrumentation and in vitro spectrophotometric methods. Good agreement was found between spectrophotometric and in situ measurements. Spatial variability in light attenuation was primarily attributable to scattering, and, to a lesser degree, by absorption due to particles. Light absorption by phytoplankton pigments was a relatively small proportion of total absorption in the blue region of the spectrum due to a strong detrital signature. Optical properties were useful predictors of constituent concentrations. Scattering at 650 nm was significantly correlated with total suspended matter concentrations. Absorption at 676 nm was significantly correlated with chlorophyll a concentrations. Phytoplankton pigment spectral absorption showed a consistent shape across a wide range of optical conditions, an indication that pigment composition changes related to photoacclimation were minor across plume gradients. These findings will be used to support complementary efforts to characterize the irradiance
A three year long field program investigating cross-margin transport processes and their effect on lake ecology was initiated in southern Lake Michigan in late 1997. An observation strategy involving both Eulerian and Lagrangian techniques is being used to describe the coastal flow. Ten current meter moorings were deployed in October 1997 and were concentrated along the southeast coast of the lake in an attempt to better resolve convergence zones and offshore/onshore transport. Six moorings were configured with a mid-level and near-bottom mounted vector averaging current meter and the four remaining moorings consisted of a bottom mounted acoustic Doppler current profiler. The longshore and crosshore barotropic current response over winter '97 - '98 revealed a dominant longshore flow with mean currents nearly an order of magnitude stronger than the crosshore component. Time series data show numerous current reversals with data from the shallow 20 m moorings showing a strong coupling with changes in the surface wind stress. Data from the 60 m moorings were more strongly influenced by the large scale circulation. A topographic wave with an approximate four day period, first identified by Saylor et al. (1980), also appears in the coastal data. The effects of the March '98 storm show pronounced offshore transport at all sites. The averaged currents show a net northerly longshore and a very weak westerly offshore flow. Surface flow characteristics were also observed using satellite reporting drifting buoys during April '98 and '99.
A major goal of the NSF-OCE and NOAA-COP on Coastal Studies in the Great Lakes is to improve the understanding of transport processes across the coastal margins of the Great Lakes. Major features of the coastal margin of the Great Lakes that affect transport include coastal upwelling and downwelling, baroclinic geostrophic coastal jets with speeds at times exceeding 50 cm/sec, thermal bars with strong vertical convection, etc. This poster describes preliminary results of numerical simulations comparing Lakes Michigan and Superior in terms of along shore and cross shore transport and flux of mass and carbon as a result of episodic events listed above. Also presented are analyses illustrating the necessity of including episodic events to balance the primary production budgets of both Lakes. The model data are compared with field data whenever possible for comparison and verification.
Numerical models of coastal circulation, wind-waves, and sediment transport are used to investigate the role of wind-induced circulation in the offshore transport of sedimentary material in Lake Michigan during the massive March, 1998 turbidity plume event. Model results showed that circulation in Lake Michigan is highly episodic since it is almost entirely wind-driven in early spring. The characteristic wind-driven circulation pattern in the lake consists of two counter-rotating gyres, a counterclockwise-rotating gyre to the right of the wind and a clockwise-rotating gyre to the left. The gyres are separated by a convergence zone along the downwind shore with resulting offshore flow and a divergence zone along the upwind shore with onshore flow. This two-gyre circulation pattern with offshore flow was especially clearly seen during a northerly wind event (with winds up to 17 m/s) in March in southern Lake Michigan. The strongest sediment resuspension occurred in the southern lake and the shallow waters along the coastline. This is because of the larger waves in southern Lake Michigan due to the dominant northerly wind in this early spring period. The circulation and wave models are driven with surface momentum flux derived from observed meteorological conditions at 12 land stations in March 1998 and also with surface winds calculated using the mesoscale meteorological model MM5 (Dudhia, 1993) in four dimensional data assimilation mode on a 6 km grid in order to determine the sensitivity of onshore-offshore transport in southern Lake Michigan to various types of wind data. Current observations from 11 subsurface moorings are compared to simulated current in order to estimate the quality of the meteorological data sets.
Particle concentrations within the coastal margin of Lake Michigan vary dramatically in response to episodic sediment resuspension events. This nearshore zone is characterized by non-depositional sand, hard clay and cobble bottoms wherein particles reside only temporarily. Changing inventories of naturally occurring short-lived radionuclides can provide information on the frequency and duration of resuspension, on the residence time of particles in the nearshore benthic system, and on the cross margin transport of particulate materials to the major zones of permanent deposition and burial on the eastern margin of the southern basin. Samples of the resuspendible sediment pool have been collected using an ROV deployed sampling system throughout the southern basin of the lake in depths from 10 to 40 meters. Inventories of the mass of resuspendible material on the bottom range from 10 to 300 mg cm-2 and inventories of excess Th-234 (half life = 24 d) range from approximately 10 to 1000 fCi cm-2. Residence times for particles in the overlying water based upon the flux of excess Th-234 to the sediment are on the order of days to weeks. In combination with water column inventories, these sediment inventories provide measurements of time dependent fluctuations in particle deposition, resuspension, particle settling rates, and alongshore transport.
Estimating the residence time of new and resuspended particles in the nearshore water column of southern Lake Michigan is a crucial objective of the EEGLE (Episodic Events - Great Lakes Experiment) program. Suspended particle residence times combined with measured and modeled vectors of advection and diffusion can be used to estimate the cross-margin (near shore - off shore) flux of nutrients and contaminants. To estimate the residence time of suspended particles, we measured 1) surface water, 2) integrated water column and 3) surface sediment activities of the particle reactive tracers Th-234 and Th-228 - giving us essentially three semi-independent residence time estimates. Based on samples collected on 10 cruises over a two year period at 14 stations ranging in depth from 10 to 150 meters, particle residence times range from less than one week to over six months. We intend to show how these residence times correlate with water column depth, external energy inputs (i.e. wind and storm activity) and biological productivity. We will also discuss the concordance of residence time estimates based on isotopic disequilibrium methods and how well these estimates agree to residence times calculated from sediment trap data.