Judith Wells Budd

A model study of the coupled biological and physical dynamics in Lake Michigan

A coupled physical and biological model was developed for Lake Michigan. The physical model was the Princeton ocean model (POM) driven directly by observed winds and net surface heat flux. The biological model was an eight-component, phosphorus-limited, lower trophic level food web model, which included phosphate and silicate for nutrients, diatoms and non-diatoms for dominant phytoplankton species, copepods and protozoa for dominant zooplankton species, bacteria and detritus. Driven by observed meteorological forcings, a 1-D modeling experiment showed a controlling of physical processes on the seasonal variation of biological variables in Lake Michigan: diatoms grew significantly in the subsurface region in early summer as stratification developed and then decayed rapidly in the surface mixed layer when silicate supplied from the deep stratified region was reduced as a result of the formation of the thermocline. The non-diatoms subsequently grew in mid and late summer under a limited-phosphate environment and then declined in the fall and winter as a result of the nutrient consumption in the upper eutrophic layer, limitation of nutrients supplied from the deep region and meteorological cooling and wind mixing. The flux estimates suggested that the microbial loop had a significant contribution in the growth of microzooplankton and hence, to the lower-trophic level food web system. The model results agreed with observations, suggesting that the

Approaching storm: Disappearing winter bloom in Lake Michigan

ABSTRACT Between 1990 and 2001, late-winter phytoplankton blooms were common in parts of the lower Great Lakes (southern Lake Michigan, Saginaw Bay and southern Lake Huron, and western Lake Erie), providing resources for over-wintering Zooplankton. In Lake Michigan up to 2001, detailed remote sensing and ship studies documented well-developed late-winter blooms in the southern gyre (circular bloom termed the ‘doughnut’). However, from 2001 to 2008, the winter blooms in Lake Michigan also supported early season veliger larvae from the introduced, cold-water adapted “profunda” morph of quagga mussels (Dreissena rostriformis bugensis). Remote sensing and ship studies revealed that settled mussels caused an extraordinary increase in water transparency and a simultaneous decrease of Chl a in the late-winter bloom. Before quagga mussels in 2001, water transparency was 74–85% at deep-water sites, whereas it increased progressively to 89% by 2006 and 94–96% by 2008. Chlorophyll a concentrations in the gyre rings were 1.1–2.6 µg/L in 2001, declining to 0.5–1.7 µg/L by 2006 and 0.4–1.5 µg/L by 2008. The reduction of Chl a in the winter bloom rings from 2001 to 2008 was 56–78% for the western limb and 74–75% for the eastern limb. Zooplankton species abundance, composition and abundance also changed, as cyclopoid copepods became very scarce and overwintering omnivorous calanoid copepods declined. Reduction in late-winter phytoplankton and Zooplankton poses a serious threat to open-water food webs.

Prognostic Modeling Studies of the Keweenaw Current in Lake Superior. Part I: Formation and Evolution

Abstract The formation and evolution of the Keweenaw Current in Lake Superior were examined using a nonorthogonal-coordinate primitive equation numerical model. The model was initialized by the monthly averaged temperaturefield observed in June and September 1973 and run prognostically under different forcing conditions with and without winds. As a Rossby adjustment problem, the model predicted the formation of a well-defined coastal current jet within an inertial period of 16.4 h after the current field adjusted to the initial temperature field. The magnitude and direction of this current jet varied with the cross-shelf temperature gradient and wind velocity. It tended to intensify during northeastward (downwelling favorable) winds, and to lessen, or even reverse, during southwestward to northwestward (upwelling favorable) or southeastward (downwelling favorable) winds. In a case with strong stratification and without external atmospheric forcings, a well-defined clockwise warm-core eddy formed near the ...

Satellite observations of Microcystis blooms in western Lake Erie

Satellite remotely sensed imagery is used increasingly to monitor algal blooms in coastal systems. Cyanobacteria and coccolithophore blooms are characteristically highly reflective and can be documented through satellite images (ACKLESON et al. 1989, .KAHRU et al. 1993, BROWN & YODER 1994) . .KAHRU et al. (1993), in particular, used a digital filter of Advanced Very High Resolution Radiometer (AVHRR) band l (visible) and bands 4-5 (thermal IR) imagery to detect cyanobacteria accumulations in the Baltic Sea. The tracking of a red tide dinoflagellate bloom off the southern coast ofNorth Carolina in 1987 (PYKE 1989, TESTER et al. 1991) resulted in the establishment of the NOAA CoastWatch Program, which distributes AVHRR data on a daily basis (LESHKEVICH et al. 1993). TYLER & STVMPF (1989) coupled thermal and turbidity information from AVHRR data and shipboard sampling to track a red tide bloom of Heterocapsa triquetra in Chesapeake Bay. More recendy, GoWER (1995) used the AVHRR to investigace plankton blooms and river plumes along the British Columbia coast. Intensive water quality monitoring efforts in Saginaw Bay during 1990-1994 (FAHNENSTIEL et al. 1995) allowed simultaneous analysis and empirical comparisons of AVHRR satellite imagery with in situ data. Increased reflectances associated with blue-green algae blooms in the bay were detected in satellite images during 1993-1994 (Buoo et al. 2001). Our objective is to estimate the area! extent of Microcystis blooms using AVHRR and Landsat Thematic Mapper (TM) imagery o f western Lake Erie. We use a digital fi!ter method similar to .KAHRU et al. (199 3) to track high reflectance water associated with Microcystis blooms in western Lake Erie during 1995. A time series data see allowed comparison of satellite-derived sediment maps (as AVHRR remote sensing reflectance, RSR) of western Lake Erie with reported observations of blooms during August-October of 1995. Changes in mean RSR and estimates of area! extent are also presented. Ana!ysis of Landsat TM imagery acquired on September 23, 1995 allowed separation of chlorophyll from suspended materials, confirming site-specific observations of Microcystis in the southwestern basin ofLake Eri e.

Influences of suspended sediments on the ecosystem in Lake Michigan: a 3-D coupled bio-physical modeling experiment

The influence of suspended sediments on the Lake Michigan ecosystem was examined using a 3-dimensional (3-D) coupled biological and physical model developed by Chen et al. (part I). The model was driven by the realistic meteorological forces observed in March 1998, with daily inputs of suspended sediment concentration that were derived from temporally and spatially interpolated satellite imagery. The model results show the significant impact of a seasonally recurring coastal resuspension plume on the spatial and temporal variation of the nutrients and plankton in southern Lake Michigan. The plume-released nutrients played an essential role in maintaining the nutrient level in the lake. Although the growth of phytoplankton in the plume depended on the availability of nutrients and light, the offshore decrease in phytoplankton biomass still satisfied the Sverdrup’s relationship. Cross-shore fluxes of nutrients and phytoplankton were controlled by episodic wind events with a period of 5–7 days: offshore during southward winds and onshore during northward winds. The flux estimates for biological variables suggest that the microbial food web is a key contributor to secondary production in southern Lake Michigan and the lower trophic level food web system could be dynamically divided into two decoupled loops: (1) detritus–bacteria–microzooplankton–large zooplankton; and (2) nutrient–phytoplankton–detritus. The model-predicted spatial distributions of nutrients and phytoplankton were in reasonable agreement with observations taken during the 1998 EEGLE interdisciplinary cruises, suggesting that the model was sufficiently robust to capture the basic characteristics of the Lake Michigan ecosystem during the plume event.

Prognostic Modeling Studies of the Keweenaw Current in Lake Superior. Part II: Simulation

The Keweenaw Current, observed along the coast of the Keweenaw Peninsula in Lake Superior during July 1973, was simulated using a 3D, nonorthogonal coordinate transformation, primitive equation coastal ocean model. The model domain covered the entire lake with a high resolution of 250‐600 m in the cross-shelf direction and 4‐6 km in the alongshelf direction along the peninsula. The model was initialized using the monthly averaged temperature field observed in June 1973 and was run prognostically with synoptic wind forcing plus monthly averaged heat flux. Good agreement was found between model-predicted and observed currents at buoy stations near Eagle Harbor. Comparison of the model results with and without inclusion of heat flux suggested that combined wind and heat fluxes played a key role in the intensification of the Keweenaw Current during summer months. The model-predicted relatively strong near-inertial oscillations occurred episodically under conditions of a clockwise-rotating wind. These oscillations intensified at the surface, were weak near the coast, and increased significantly offshore.

Assessing the application of SeaWiFS ocean color algorithms to Lake Erie

ABSTRACT The feasibility of satellite-based monitoring of phytoplankton chlorophyll a concentrations in Lake Erie is assessed by applying globally calibrated, ocean-derived color algorithms to spatially and temporally collocated measurements of SeaWiFS remote sensing reflectance. Satellite-based chlorophyll a retrievals were compared with fluorescence-based measurements of chlorophyll a from 68 field samples collected across the lake between 1998 and 2002. Twelve ocean-derived color algorithms, one regional algorithm derived for the Baltic Seas Case 2 waters, and a set of regional algorithms developed for the western, central and eastern basins of Lake Erie were considered. While none of the ocean-derived algorithms performed adequately, the outlook for the success of regionally calibrated and validated algorithms, with forms similar to the ocean-derived algorithms, is promising over the eastern basin and possibly the central basin of the lake. In the western basin, each of the regional algorithms considered performed poorly, indicating that alternative approaches to algorithm development, or to satellite data screening and analysis procedures will be needed.

Satellite-based Sediment and Chlorophyll a Estimates for Lake Superior

Abstract Estimates of chlorophyll a and total suspended matter (TSM) concentrations are obtained from Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) imagery. We used a coastal atmospheric correction involving an iterative correction for absorbance in the near-infrared bands; however, most scenes had negative radiances at 412 nm, necessitating application of a form of absorbing aerosol correction. An empirical relationship between remote sensing reflectance at 555 nm and TSM concentrations was established, with linear covariance R 2 = 0.74. The NASA coastal bio-optical algorithm (OC2-V2) for chlorophyll a had strong co-variance with ground-based chlorophyll a (R 2 = 0.87), but with a 3-fold overestimate in chlorophyll a concentration. Satellite-based chlorophyll a concentrations (C OC2 ) were corrected based on the regression equation to provide a new solution as follows: C OC2 (transform) = 0.3(C OC2 -0.4). The overall result for Lake Superior is a regional chlorophyll a algorithm, analogous to algorithms that have been developed for other coastal regions of the U.S.

Evaluation and Regional Optimization of Bio-optical Algorithms for Central Lake Superior

Bio-optical properties of the ocean have been broadly investigated using Coastal Zone Color Scanner (CZCS) and Sea-viewing Wide Field-of-view Sensor imagery (SeaWiFS), from which numerous globally and regionally optimized bio-optical algorithms have emerged. As a first step in evaluating bio-optical algorithms for the Laurentian Great Lakes, we tested ten published marine bio-optical algorithms (9 empirical and 1 semi-analytical), in central Lake Superior using optical data and discrete water samples collected in 1998–2000. Although the semi-analytical algorithm provided a better result than the empirical algorithms, all of the algorithms overestimate chlorophyll a concentration. We derived optimized empirical parameters using two of the best algorithms (Ocean Color-Version 4 (OC4) and semi-analytical algorithms), but the models do not simulate chlorophyll a concentrations well. Compared to other coastal waters, the chlorophyll a concentration in Lake Superior is low (0.2–1.5 μg L−1) and the fraction of total absorbance attributable to chromophoric dissolved organic matter (CDOM) is high. The ratio of absorbance due to chlorophyll a to that of CDOM ranges from 0.001 to 0.262 between 443 and 555 nm in Lake Superior, which is 2–3 times lower than in case 1 waters, and indicates that semi-analytical, as opposed to strictly empirical, bio-optical algorithms may provide better estimates of chlorophyll a concentration in Lake Superior.

Large-scale Transport Phenomena in the Keweenaw Region of Lake Superior: the Ontonagon Plume and the Keweenaw Eddy

Large-scale transport phenomena in the Keweenaw region of Lake Superior are examined using multi-sensor satellite remote sensing. Satellite-based estimates of chlorophyll a (COC2) and total suspended matter (TSMSAT) concentrations are obtained from Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) imagery, while lake surface temperatures (LST) are obtained from Advanced Very High Resolution Radiometer (AVHRR) imagery. Analysis of 2 years of temperature, chlorophyll, and sediment maps reveal several potentially important features in central Lake Superior. SeaWiFS TSMSAT images indicate the presence of an offshore sediment plume originating near Ontonagon, Michigan that was observed on multiple occasions in March, April, and May of both years. Plume materials were observed northeast-ward of Ontonagon at distances of 80–100 km offshore, although reversals in the pattern were observed. During summer months, the Keweenaw current culminated in a clockwise rotating eddy east of the tip of the Keweenaw Peninsula. The Keweenaw eddy, identified in both AVHRR LST and SeaWiFS COC2 maps, was 50–75 km in diameter, and it changed shape and location during July, August, and September of both years. In August and September 1999, warm waters originating near the southern shore of Isle Royale and in central Lake Superior extended in a southeasterly direction toward the tip of the Keweenaw and were subsequently entrained in the Keweenaw eddy. The physical mechanisms that create the eddy and its potential biological significance for lower trophic food web interactions are not understood at this time.