Lead in Lake Michigan and Green Bay surficial sediments

Sediment samples were collected in 1987–1990 from Green Bay and in 1994–1996 from Lake Michigan. Surficial sediments (0–1 cm) from both locations were analyzed for lead for the purpose of describing the horizontal variation of lead in 1994–1996 Lake Michigan and 1987–1990 Green Bay sediments, estimating lead fluxes to surficial sediments, and comparing results to earlier studies. With Lake Michigan concentrations ranging from below the method detection limit to 180 μg/g, the surficial sediments had mean and median lead concentrations of 70 μg/g and 64 μg/g, respectively. Lead concentrations in Green Bay surficial sediments were similar to those in Lake Michigan and ranged between the method detection limit and 160 μg/g. For the bay, mean and median concentrations were 58 and 59 μg/g, respectively. Surficial lead concentrations were highest in the Southern, Waukegan, and Grand Haven basins of Lake Michigan and in the central region of Green Bay in the vicinity of Chambers Island. For Lake Michigan and Green Bay, dated sediment cores illustrate the decline in lead concentrations during the last 30 and 10 years, respectively. Lead fluxes ranged between < 0.049 and 7.2 μg/cm²/yr for Green Bay and between 0.47 and 20 μg/cm²/yr for Lake Michigan. Lead fluxes to Lake Michigan were lower than those reported for 1972. These are the most comprehensive fluxes of lead to Lake Michigan and Green Bay surficial sediments reported to date.     

Green Bay,Lake Michigan: A proving ground for Great Lakes restoration

Green Bay has sometimes been referred to as the largest freshwater “estuary” in the world. Its watershed, much of it in intensive agriculture, comprises one-third of the Lake Michigan basin and delivers one-third of the lake's total phosphorus load. At one time, the major tributary, the Fox River, was considered the most heavily industrialized river in North America, primarily from paper manufacturing. Deterioration in water quality and the loss of beneficial and ecological uses have been extensive and began well back into the last century. More recently, the bay has also become a test case for our resolve to remediate and restore ecosystems throughout the Great Lakes and elsewhere. Green Bay has stimulated a significant amount of widely relevant research on the fate and behavior of toxics, biogeochemistry, habitat, biodiversity, and ecological processes. The bay represents a true “proving ground” for adaptive restoration. Key findings of the recent summit on the Ecological and Socio-Economic Tradeoffs of Restoration in the Green Bay Ecosystem are summarized here. Foremost among recommendations of the workshop was the creation of a “Green Bay Ecosystem Simulation and Data Consortium” serving as a data clearing house, building upon the significant progress to date, and developing a modeling framework and visualization tools, furthering public outreach efforts, and ensuring a sustained growth in scientific expertise. Funding was estimated to be on the order of ~$15–20M over the next ~5?years – a modest investment relative to the value of the ecosystem and the long-term cost of inaction.     

Spatial analysis of toxic or otherwise bioactive cyanobacterial peptides in Green Bay,Lake Michigan

Cyanobacterial harmful algal blooms (cyanoHABs) are a growing problem in freshwater systems worldwide. CyanoHABs are well documented in Green Bay, Lake Michigan but little is known about cyanoHAB toxicity. This study characterized the diversity and spatial distribution of toxic or otherwise bioactive cyanobacterial peptides (TBPs) in Green Bay. Samples were collected in 2014 and 2015 during three cruises at sites spanning the mouth of the Fox River north to Chambers Island. Nineteen TBPs were analyzed including 11?microcystin (MC) variants, nodularin, three anabaenopeptins, three cyanopeptolins and microginin-690. Of the 19 TBPs, 12 were detected in at least one sample, and 94% of samples had detectable TBPs. The most prevalent TBPs were MCRR and MCLR, present in 94% and 65% of samples. The mean concentration of all TBPs was highest in the Fox River and lower bay, however, the maximum concentration of all TBPs occurred in the same sample north of the lower bay. MCs were positively correlated with chlorophyll and negatively correlated with distance to the Fox River in all cruises along a well-established south-to-north trophic gradient in Green Bay. The mean concentration of MC in the lower bay across all cruises was 3.0?±?2.3?μg/L. Cyanopeptolins and anabaenopeptins did not trend with the south-north trophic gradient or varied by cruise suggesting their occurrence is driven by different environmental factors. Results from this study provide evidence that trends in TBP concentration differ by congener type over a trophic gradient.     

The Role of Sediments in the Nitrogen Budget of Lower Green Bay,Lake Michigan

The extreme southern portion of Green Bay is a shallow (1 to 5 m depth), eutrophic water body which receives considerable nutrients from the Fox River and metropolitan Green Bay, Wisconsin. Research to evaluate the effect of sediments on nitrogen (N) in the bay entailed periodic sampling of waters and sediments at six sites over 20 months and laboratory investigations of the rates of nitrification, denitrification, mineralization, immobilization, and N₂ fixation. The monitoring data indicated that the N concentrations, approximately 0.6 and 0.8 mg/L of inorganic and organic N, respectively, in the bay waters are considerably higher than the threshold limits that may cause algal bloom and aquatic weed problems. Consideration of the available sediment N pool with respect to recognizable N inputs indicated that only 1.2 percent of the yearly N loading from the Fox River is present in the active sediment layer. Nitrification and subsequent denitrification at the sediment-water interface as a result of intermittent wind stirring could be a major sink for N, but presently it has a minor impact due to the high loading rate of N in this ecosystem. The study indicates that even if approximately 50 percent of the present point source loading of N were eliminated by pollution abatement, the N input from nonpoint sources (combined with existing concentrations of phosphorus in the bay waters) would be sufficient to maintain eutrophic conditions.     

Great Lakes total phosphorus revisited: 2. Mass balance modeling

Mass balance models are used to simulate chloride and total phosphorus (TP) trends from 1800 to the present for the North American Great Lakes. The chloride mass balance is employed to estimate turbulent eddy diffusion between model segments. Total phosphorus (TP) concentrations are then simulated based on estimated historical and measured TP loading time series. Up until about 1990, simulation results for all parts of the system generally conform to measured TP concentrations and exhibit significant improvement due primarily to load reductions from the Great Lakes Water Quality Agreement. After 1990, the model simulations diverge from observed data for the offshore waters of all the lakes except Lake Superior with the observations suggesting a greater improvement than predicted by the model. The largest divergence occurs in Lake Ontario where the model predicts that load reductions should bring the lake to oligo-mesotrophic levels, whereas the data indicate that it is solidly oligotrophic and seems to be approaching an ultra-oligotrophic state. Less dramatic divergences also occur in the offshore waters of lakes Michigan, Huron and Erie. In order to simulate these outcomes, the model's apparent settling velocity, which parameterizes the rate that total phosphorus is permanently lost to the lake's deep sediments, must be increased significantly after 1990. This result provides circumstantial support for the hypothesis that Dreissenid mussels have enhanced the Great Lakes phosphorus assimilation capacity. Finally, all interlake mass transfers of TP via connecting channels have dropped since phosphorus control measures were implemented beginning in the mid-1970s.     

Variations in chemical speciation and reactivity of phosphorus between suspended-particles and surface-sediment in seasonal hypoxia-influenced Green Bay

Water, suspended-particles, and surface-sediment samples were collected from Green Bay, Lake Michigan, for the measurements of phosphorus (P) species, including dissolved/particulate-P, inorganic/organic-P, and five different forms of particulate-P, namely exchangeable- or labile-P (Ex-P), iron-bound-P (Fe-P), biogenic-apatite and/or CaCO₃-associated-P (CFA-P), organic-P (Org-P) and detrital-apatite-P (Detr-P) to elucidate their reactivity and transformation pathways in the water column. Suspended particles contained mainly Ex-P (25?±?15%), Fe-P (28?±?12%) and Org-P (29?±?7%). In contrast, Detr-P (34?±?10%) and Org-P (36?±?12%) were the predominant P species in surface sediment. Contents of Ex-P, Fe-P, Org-P and CFA-P decreased consistently from suspended-particles to surface-sediment, but an increase was observed for the Detr-P, indicating a net loss of Ex-P, Fe-P, Org-P and CFA-P from particulate into dissolved phase. Such active regeneration of P in the water column between particulate and dissolved phases may serve as an internal phosphate source in Green Bay, especially under hypoxic conditions. Degradation of organic matter in south central bay areas seemed to promote hypoxia and enhance the reductive-dissolution of Fe-P and preservation of Org-P under low-oxygen conditions in the central bay. Overall, Ex-P, Fe-P, CFA-P and Org-P species, which comprised up to 50–90% of total particulate-P, can be collectively considered as potentially-bioavailable-P (BAP). Under low-phosphate (0.022?±?0.014?μM in Green Bay) and summer low-oxygen/hypoxic conditions, suspended-particles may release up to 71% of their BAP before deposited in sediment although the BAP regeneration decreased along the south-north transect in Green Bay.     

The Green Bay saga: Environmental change,scientific investigation,and watershed management

The Green Bay watershed, draining a total area of approximately 40,468?km², comprises about a third of the Lake Michigan drainage. In the early years, fur trade was the dominant economic activity within the watershed. Later, when timber harvesting, papermaking, and agriculture came on the scene in the 19th and early 20th centuries, major environmental changes occurred in a relatively short period of time. Nutrient and sediment loadings, accompanied by organic wastes from sawmills and paper mills, resulted in a pollutant overload in the Fox River and in the eutrophication of the waters of lower Green Bay. Citizen complaints about these severely degraded conditions initiated a period of scientific investigation. Starting slowly with a few studies and surveys in the first half of the 20th century, serious investigatory work began at mid-century with support from the University of Wisconsin Sea Grant Institute. Examples of topics that have been investigated since then with support from numerous sources are: biological oxygen demand (BOD), phosphorus and total suspended solids loads, trophic status and food chain efficiencies, coastal wetland characterization, dynamics of the benthic layer, algae and abiotic solids, phosphorus cycling and mass balance, PCBs, seasonal hypoxia, and climate change impacts. These studies have provided the scientific foundation for government-led programs such as the Green Bay Remedial Action Program, the PCB clean-up program, and the TMDL program. Progress has been made—reduction in BOD is an example—but a fuller rehabilitation of this large-scale ecosystem remains an elusive goal. The saga goes on.     

Calibration of a Hydraulic Transport Model for Green Bay,Lake Michigan

The U.S. Environmental Protection Agency (USEPA) has developed and implemented a comprehensive plan for modeling the transport and fate of toxic chemicals in Green Bay, Lake Michigan. Characterization of the hydrodynamic behavior of Green Bay was a key step in the model calibration process. To accomplish this, the WASP4 finite-segment model framework was calibrated to describe chloride dynamics during an 18-month period (Jan 1989 to May 1990). Extensive use was made of field data (currents, water temperature, wind, and ice cover), and new interpretations of these data were developed. The direction of circulation in Green Bay typically reverses every three to five days during open water periods; these reversals are closely correlated to wind direction relative to the axis of the Bay. Inflows from Lake Michigan exceed the combined inflows of all tributaries by over twenty times; the circulation of these inflows through Green Bay are greatly enhanced each spring when the ice cover melts. During open water periods, bulk dispersion coefficients typically range from 20 to 120 m²/s in the longitudinal direction, and from 5 to 40 m²/s in the transverse direction. The calibrated advection and dispersion fields accurately describe the major hydrodynamic features of Green Bay, and were incorporated into input data sets for models of eutrophication, organic carbon sorbents, and toxic chemicals.     

Comment on “A Model for Total Phosphorus in Saginaw Bay”

The subject paper (Canale and Squire 1976) presents the results of the application of a steady-state mass balance model to chloride and total phosphorus data for Saginaw Bay in 1974. Richardson (1974, 1976) has modeled chloride in Saginaw Bay on both a steady-state and a time variable basis. In this work, the steady-state model was shown to be inadequate for this system, except during stable periods in summer and fall. Since most of the loading of pollutants to the bay does not occur at these times, the time variable model was developed. The purpose of this comment is to compare similar aspects of these projects and to contrast the steady-state and time variable approaches with respect to their ability to simulate observed phenomena and the utility of their results.     

Quantification of internal phosphorus load in large,partially polymictic and mesotrophic Lake Simcoe,Ontario

Hypoxia and cyanobacteria still occur occasionally in large, mesotrophic Lake Simcoe, and total phosphorus (TP) concentration has remained relatively constant despite external nutrient load reduction. This may indicate a potential internal P source. Internal load as redox-dependent P release from bottom sediments is hard to determine in such a relatively shallow and mostly mixed lake. This study represents the first attempt to quantify internal P loading over many years for the three main sections of Lake Simcoe. Internal load was determined (a) as in situ estimate based on TP increases between July and October and (b) as gross estimate from the product of experimentally determined P release rates and hypoxic extent of sediment surfaces in space and time. Hypoxic extent was quantified (1) as the hypoxic factor determined from dissolved oxygen profiles below the level of 3.5 mg/L, and (2) as active sediment area release factor (AA) modeled from summer euphotic TP concentration, which is especially useful in the mixed sections. Annual internal load for the whole lake was determined as a near constant 62.2 metric tonnes/yr (86 mg/m²/yr) for 1980–2011 using the gross estimates of the AA approach and 88 t/yr before and 53 t/yr after external load abatement and zebra mussel invasion using in situ estimates. Means of in situ and AA-based estimates for 2000–2011 are in close agreement except for polymictic Cook's Bay. These estimates are 45 to 89% of external load, which suggests that internal loading is an important source of P in Lake Simcoe.     

A comparative examination of recent changes in nutrients and lower food web structure in Lake Michigan and Lake Huron

The lower food webs of Lake Huron and Lake Michigan have experienced similar reductions in the spring phytoplankton bloom and summer populations of Diporeia and cladocerans since the early 2000s. At the same time phosphorus concentrations have decreased and water clarity and silica concentrations have increased. Key periods of change, identified by using a method based on sequential t-tests, were 2003–2005 (Huron) and 2004–2006 (Michigan). Estimated filtration capacity suggests that dreissenid grazing would have been insufficient to directly impact phytoplankton in the deeper waters of either lake by this time (mid 2000s). Despite some evidence of decreased chlorophyll:TP ratios, consistent with grazing limitation of phytoplankton, the main impact of dreissenids on the offshore waters was probably remote, e.g., through interception of nutrients by nearshore populations. A mass balance model indicates that decreased phosphorus loading could not account for observed in-lake phosphorus declines. However, model-inferred internal phosphorus dynamics were strongly correlated between the lakes, with periods of increased internal loading in the 1990s, and increased phosphorus loss starting in 2000 in Lake Michigan and 2003 in Lake Huron, prior to dreissenid expansion into deep water of both lakes. This suggests a limited role for deep populations of dreissenids in the initial phosphorus declines in the lakes, and also suggests a role for meteorological influence on phosphorus dynamics. The high synchrony in lower trophic level changes between Lake Michigan and Lake Huron suggests that both lakes should be considered when investigating underlying causal factors of these changes.     

Currents and Temperatures in Green Bay,Lake Michigan

Current velocities and water temperatures were measured in the four main passages between Green Bay and Lake Michigan and at several sites within the bay during summer and fall 1977. Monthly resultant currents indicate there is anticlockwise circulation in the bay during dominant southwesterly wind and a reversal of this pattern during episodes of northeasterly wind. It is common for two layers to flow through the mouth of the bay in opposite directions during the stratified season. Cold hypolimnetic lake water entering through the mouth and extending far into the bay maintains stratification and promotes flushing. The effects of resonance of forced and free long wave disturbances are prominent in current records; these oscillations are coherent and in phase across the mouth.     

Long-Term Simulation of PCB Export from the Fox River to Green Bay

A mass balance approach was used to model long-term PCB transport in the Fox River (Wisconsin) from Lake Winnebago to Green Bay. The objectives of this research were to (1) extend the modeling approach for the Fox River to permit realistic long-term simulations of contaminant transport and fate, (2) forecast long-term PCB export from the Fox River to Green Bay, and (3) develop a rational approach for evaluating sediment remediation alternatives. Field data collected as part of the Green Bay Mass Balance Study during 1988-90 and additional data collected by the Wisconsin Department of Natural Resources and the U.S. Geological Survey during 1992-93 were used to develop the model. A 10-year hindcast was conducted to confirm long-term model predictions. A series of 25-year forecasts were then conducted to explore the potential effects of hydrograph structure, extremely high flows, and sediment remediation on long-term PCB export from the Fox River to Green Bay. PCB export from the Fox River is forecast to decrease, and most (75%) of the PCB reservoir in Fox River sediment is expected to remain in place. However, extremely high flows in future years are forecast to cause significant PCB resuspension and export. Model forecasts suggest that long-term PCB export is only mildly sensitive to changes in hydrograph structure. Sediment remediation is forecast to reduce but not eliminate PCB export.     

Landscape-scale modeling of water quality in Lake Superior and Lake Michigan watersheds: How useful are forest-based indicators?

The Great Lakes watersheds have an important influence on the water quality of the nearshore environment, therefore, watershed characteristics can be used to predict what will be observed in the streams. We used novel landscape information describing the forest cover change, along with forest census data and established land cover data to predict total phosphorus and turbidity in Great Lakes streams. In Lake Superior, we modeled increased phosphorus as a function of the increase in the proportion of persisting forest, forest disturbed during 2000–2009, and agricultural land, and we modeled increased turbidity as a function of the increase in the proportion of persisting forest, forest disturbed during 2000–2009, agricultural land, and urban land. In Lake Michigan, we modeled increased phosphorus as a function of ecoregion, decrease in the proportion of forest disturbed during 1984–1999 and watershed storage, and increase in the proportion of urban land, and we modeled increased turbidity as a function of ecoregion, increase in the proportion of forest disturbed during 2000–2009, and decrease in the proportion softwood forest. We used these relationships to identify priority areas for restoration in the Lake Superior basin in the southwestern watersheds, and in west central and southwest watersheds of the Lake Michigan basin. We then used the models to estimate water quality in watersheds without observed instream data to prioritize those areas for management. Prioritizing watersheds will aid effective management of the Great Lakes watershed and result in efficient use of restoration funds, which will lead to improved nearshore water quality.     

Resurrection of the Lake Michigan Eutrophication Model,MICH1

The Lake Michigan model, MICH1, was developed more than 30 years ago. This framework was evaluated using field data collected in 1976 and was later applied to predict total phosphorus and phytoplankton concentrations in Lake Michigan during the 1980s and early 1990s. With a renewed interest in the interaction of phytoplankton with toxics and the applicability to Total Maximum Daily Load studies, several new models have been developed and older models have been revived. As part of our interest in plankton dynamics in Lake Michigan, the MICH1 model was resurrected. The model was evaluated over the 1976–1995 period, with a surprisingly good model fit to lake-wide average total phosphorus (TP) field data. However, the model was less successful in mimicking the chlorophyll-a measurements, especially in the hypolimnion. Given the results, the model was applied to perform a few long-term TP model simulations. Using the model with average 1994–95 phosphorus loadings, a steady state was reached within approximately 20 years, and the lakewide phosphorus concentration was below the International Joint Commission water quality guideline of 7 μg/L. This exercise demonstrated that a relatively simple, four-segment model was able to mimic the TP lake-wide data well. However, this model was less suitable to predict future chlorophyll-a concentrations due to the limitation in the representation of the foodchain and the difficulty of the coarse segmentation of the model to capture the deep chlorophyll-a layer. Strengths and limitations of this model can guide future development of eutrophication models for Lake Michigan and the other Great Lakes.     

Evidence of persistent,recurring summertime hypoxia in Green Bay,Lake Michigan

Six years (2009–2015) of temperature and dissolved oxygen profile data show hypoxic conditions are common in the bottom waters of southern Green Bay, Lake Michigan during the summer. Depleted oxygen concentrations (<5?mg?L^?1) affect nearly 70% of the 38 stations sampled representing an area of ~500–600?km². Stratification typically lasts 2+?months, from late June to early September, and some stations exhibit bottom water hypoxia (<2?mg?L^?1) at a frequency of nearly 25% when sampled during this period. A monitoring program initiated in 1986 by the Green Bay Metropolitan Sewerage District has provided a 23?year, recreational season record (May–September) of continuous (15?min interval) in situ bottom water oxygen and temperature measurements at the Entrance Light station of the Green Bay navigational channel. The duration of the hypoxic season ranges from 2?weeks to over 3?months at this shallow 7?m offshore site. This variability likely results from a combination of thermal stratification, oxygen consumption in deeper waters of the bay, and physical forcing mechanisms that drive cool, oxygen depleted, bottom waters on a southerly trajectory across this sensor. These data suggest the duration of hypoxic conditions may have increased during the stratified season in recent years. Hypoxia in the bay would also appear to be sensitive to relatively small changes in these forces, particularly changes in organic carbon loading and the duration of stratification.     

Water Quality Modeling of Lake Michigan and Consideration of the Anomalous Ice Cover of 1976–1977

An intensive survey of water quality parameters was conducted on Lake Michigan during 1976 and 1977. A dynamic phytoplankton simulation model (MICH1) was developed to investigate the observed field data and to use in forecasting lake responses to various phosphorus loading scenarios. The 1977 data indicated that the southern basin of Lake Michigan lost up to 3 μg P/ L compared to concentrations observed in 1976. In an attempt to simulate this rapid depletion of phosphorus, MICH1 required an implicit representation of suspected effects of an extensive ice cover observed during the winter of 1976–77. This included increasing the net apparent settling rate eight fold during ice cover. A total phosphorus model (TPM) was used in conjunction with MICH1 ;for forecasting. These forecasts indicate a steady-state total phosphorus concentration of 7 μg P/ L, given a target load recommended by the 1978 Water Quality Agreement. The projected time to obtain 95% of steady-state response to a load change was 7–14 years.     

Timescales of transport through Lower Green Bay

Contaminated sediments, poor water quality, and lost or altered habitat in Lower Green Bay and Fox River in the 1980s led to its listing as an Area of Concern by the International Joint Commission. Previous studies on the geophysics and health of the bay demonstrated the need for estimates of transport timescales. Such estimates can contribute to improved understanding of the transport and fate of nutrients, contaminants, and biogeochemical processes in the bay. This study reviews definitions of residence time and flushing time, estimates the temporal distribution of flushing time, the spatial and temporal distributions of residence time, and horizontal diffusivities. The study used a previously developed hydrodynamic model, a drifter experiment, a lake particle transport model, and methods appropriate for the estimation of transport timescales. The estimated residence time for lower Green Bay, 56 ± 16 days, is smaller than a previous estimate of 190 days for the whole bay, and had values similar to the estimated flushing times. The closeness between those timescales demonstrates the important role of water exchange across the Chambers Island transect. Flushing time and residence time do not follow the same trends in monthly variability because the former depends directly on water exchange across Chambers Island, while the later depends, in addition, on tributary inflows and the circulation patterns in the bay. The study includes a discussion of the relations between the timescales of transport and the previous studies of biogeochemical processes, such as trophic conditions, spatial distribution of cyanobacteria, cold-water intrusions, and hypoxia.     

Fate of phosphorus from a point source in the Lake Michigan nearshore zone

Nutrient loading into Lake Michigan can produce algal blooms which in turn can lead to hypoxia, beach closures, clogging of water intakes, and reduced water quality. The Great Lakes Water Quality Agreement targets for Lake Michigan are 5600 MT annually for total phosphorus (TP) loading, 7 μg L^?1 lake-wide mean TP concentration, and a chlorophyll-a concentration of 1.8 μg L^?1. However, in light of the recent resurgence of nuisance algal (Cladophora sp.) growth in the nearshore zone, the validity of these targets is now uncertain. The occurrence and abundance of Cladophora in the nearshore area depends primarily on the availability of dissolved phosphorus, light, and temperature. The availability of dissolved phosphorus is a potentially useful indicator of nearshore areas susceptible to excessive Cladophora growth and impaired water quality. Regulating agencies are looking for guidance in determining phosphorus loading rates that minimize local exceedance of the lake target concentration. In this study, the lake assimilative capacity was quantified by applying a biophysical model to estimate the area required for mixing and diluting wastewater treatment plant outfall TP loadings to the level of the lake target concentration during the Cladophora growing season. Model results compared well with empirical measurements of particulate and dissolved phosphorus as well as Cladophora biomass and phosphorus content. The model was applied to test scenarios of wastewater treatment plant phosphorus loading in two different years, in order to help establish phosphorus discharge limits for the plant.     

Resolution of optical gradients and pursuit of optical closure for Green Bay,Lake Michigan

Optical properties have fundamental importance to water quality, ecology, and remote sensing initiatives. Paired measurements of optically active constituents (OACs), and inherent optical properties (IOPs) and apparent optical properties (AOPs), were made in September 2010 across the optical gradients of Green Bay, extending from the Fox River to Sturgeon Bay (8 sites), and for three near-shore locations in the main basin of Lake Michigan. The array of laboratory and in situ measurements provided a robust characterization of the underwater and emergent light fields of these waters with respect to magnitudes and spectral features of the OACs, IOPs and AOPs. These measurements resolved the character and possible origins of the major gradients within the bay (5 to 10-fold differences) and the substantial differences between the bay and the main basin. The credibility of the characterizations was supported through closure analyses which demonstrated: (1) the approach to equivalence between various field and laboratory measurements, and (2) good matches of AOP observations by values predicted from measured IOPs using accepted radiative transfer expressions. The bay was demonstrated to be an optically complex case 2 system, with uncoupled variations along the spatial gradient(s) in OACs of phytoplankton biomass, colored dissolved organic material, and non-algal particulates. The documented spatial differences in optical properties rival those reported in much larger marine surveys. Radiative transfer expressions are used to predict changes in AOPs of the downwelling (underwater) attenuation coefficient and remote sensing signal in response to scenarios of changes in levels of OACs of potential ecological and management interest.