Depth Of Photic Zone Research Articles

A Regional Algorithm for Calculating the Photic Zone Thickness from the Vertical Profile of the Beam Attenuation Coefficient on the Example of the Northwestern Part of the Weddell Sea

A method is proposed for constructing a regional algorithm for calculating the thickness of the euphotic zone from in situ measurements of the vertical profiles of the beam attenuation coefficient under the condition of one-parameter optical properties of water (Case-1 water). Using the proposed methodology, a regional algorithm for determining the thickness of the photic zone was developed based on the results of synchronous measurements of the beam attenuation coefficient and photosynthetically active radiation, performed in January 2022 as part of the 87th cruise of the R/V Akademik Mstislav Keldysh for the northwestern part of the Weddell Sea. For this region, an equation was obtained that makes it possible to estimate the photic zone depth with a relative reconstruction error of 18%.

Inherent Optical Properties based Vulnerability Assessment of Euphotic Zone Compression in peatland influenced Southeast Asian coastal waters

Underwater light availability is a crucial aspect for the ecological functioning of shallow water bodies. Light extinction from terrestrial inputs is a growing threat to these coastal habitats. The blended quasi-analytical algorithm (QAA) was extended for the derivation of colored dissolved organic matter (CDOM) absorption coefficient along with other inherent optical properties (IOPs) from satellite observations for Southeast Asian waters. The contribution of these IOPs to diffuse attenuation of light (Kd) and penetration depth (Zd) was investigated. A vulnerability assessment was performed to identify locations potentially threatened by poor light quality in Southeast Asian waters. Advection of peatland-influenced Sumatran coastal waters rich in organic matter (ag(400nm): 1.0-2.0m-1) and sediments (bbp(400nm): 0.5-1m-1) drive the spatial heterogeneity of Sunda shelf seawater. Photic zone depth, Zd(490nm), is year-round restricted to ≤5m for critically vulnerable Sumatran coastal waters (vulnerability index, VI>0.8). This critically vulnerable state is further extended towards the southern Malacca Strait, influencing the eastern Singapore Strait from June to September. The areas harbouring marine ecosystems in the shelf waters attain a higher threshold (VI=0.6-0.8), constraining the photosynthesis to depths ≤10m. A transformation of central Malacca Strait from not vulnerable (VI<0.2) to highly vulnerable (VI=0.6-0.8) state from June to September indicates poor light conditions. Further increases in CDOM and sediment inputs into these water columns, therefore, constitute a clear risk of reducing light availability, which may have damaging effects on the functioning of coastal habitats. This study underscores the need for a complete ecological risk assessment for Southeast Asia to aid in the effective management of marine ecosystems.

Distribution of calanoid copepods across the mesoscale frontal zone of tropical-subtropical convergence off México

The distribution of calanoid copepods was investigated in the convergence zone between the California Current Water and Transitional Tropical Water in June 2010 to understand how this zooplankton community responds to mesoscale environmental structure. Hydrographic conditions and zooplankton profiles were measured along two transects which crossed a mesoscale front and adjacent structures, visible in sea surface temperature and ocean color satellite images. The hydrographic structure showed strong temperature (~2 °C) and oxygen (50 μmol kg−1) gradients in the frontal zone (~90 km) associated with a sinking of the thermocline and oxycline, from ~30 m depth in the Transitional Tropical Water, to ~ 90 m depth in the California Current Water. The front was marked by a salinity minimum (<34.3 g kg−1) that sank along the sloping thermocline. High chlorophyll-a concentrations associated with the thermocline were detected in the Transitional Tropical Water, where a cyclonic eddy was defined. These concentrations decreased as the thermocline depth increased. Multivariate analysis defined three calanoid copepod habitats related to the hydrographic structure. A habitat with the highest copepod concentration (24,704 copepods 1000 m−3) was found on the tropical water side associated with a cyclonic eddy. Subeucalanus subtenuis and Centropages furcatus, indicator species, were concentrated between the thermocline (~30 m depth) and the sea surface. On the subtropical water side, where the thermocline was deeper, a habitat with the lowest copepod concentration (3304 copepods 1000 m−3, an order of magnitude smaller), characterized by Euchaeta indica and Centropages elongates, was observed. As an unexpected result, the habitat located in the frontal zone, characterized by Scolecithrix bradyi and Heterorhabdus papilliger had low copepod concentration (4453 copepods.1000 m-3). This low concetration could be due to the steep thermocline slope in the frontal zone, which could cause the thermocline depth to rapidly exceed the photic zone depth, and therefore affect the availability of food for calanoid copepods. It is concluded that the definition of the calanoid copepod habitats, was not only caused by the front itself, but by its interaction with other fine-scale mechanisms that generate oceanographic conditions leading to development of different food webs depending on the structure of water column.

Refining species distribution model outputs using landscape-scale habitat data: Forecasting grass carp and Hydrilla establishment in the Great Lakes region

Abstract Forecasts of the locations of species invasions can improve by integrating species-specific climate and habitat variables and the effects of other invaders into predictive models of species distribution. We developed two species distribution models (SDMs) using a new algorithm to predict the global distributions of two nonindigenous species, grass carp ( Ctenopharyngodon idella ) and Hydrilla ( Hydrilla verticillata ), with special attention to the North American Great Lakes. We restricted the projected suitable habitat for these species using relevant habitat data layers including accumulated Growing Degree Days (GDD), submersed aquatic vegetation (SAV), wetlands, and photic zone. In addition, we restricted the grass carp niche by the projected Hydrilla niche to explore the potential spatial extent for grass carp given a joint invasion scenario. SDMs showed that climate conditions in the Great Lakes basin were often suitable for both species, with a high overlap between the areas predicted to be climatologically suitable to both species. Restricting Hydrilla regions by GDD and photic zone depth showed that the nearshore zones are primary regions for its establishment. The area of predicted habitat for grass carp increased greatly when including Hydrilla niche as a potential habitat for this species. Integrated risk maps can provide a means for the scientifically informed prioritization of management resources toward particular species and geographic regions.

Summer nutrient dynamics and biological carbon uptake rate in the Changjiang River plume inferred using a three end-member mixing model

A three end-member (Changjiang River Plume, CRP; Outer-shelf Surface Water, OSW; and Outer-shelf Deep Water, ODW) mixing model based on quasi-conservative temperature and salinity was adopted to identify the relative contribution of different water masses to nutrient inventory and to estimate biological nutrient uptake in the plume-impacted area by considering the difference between the model-predicted and ambient concentration of nutrients up to a depth of PZD10 (photic zone depth to 10% of surface photosynthetically active radiance (PAR)). The end-member composite suggested that the mixing of CRP and OSW was the main process regulating nutrient dynamics and phytoplankton growth, while the correlation of ODW with ΔP indicated that the outcropped upwelling water contributed to the replenishment of P leading to Chl a accumulation to some extent. The ratio ΔDIN:ΔP:ΔSi:excess O2 in the euphotic zone where excess O2>10μmolL−1 was estimated to be 28±9:1:33±13:145±71.A simple box model was used to evaluate biological carbon uptake rate in the euphotic zone based on nutrient deviation, Redfield ratio (6.6 C:1 DIN), and residence time of nutrients, assuming that the Changjiang River was the unique source of nutrients in the quasi-static box. The biological carbon uptake rates derived from the DIN, P and Si deviation were 465, 344, and 626 mg Cm−2d−1, respectively; these values were comparable to the POC flux (486±275mg Cm−2d−1) derived from sediment trap. This finding suggested that the Changjiang River Plume was responsible for phytoplankton growth and subsequent high POC flux out of the euphotic zone. Furthermore, the community respiration rate was estimated to be 634mg Cm−2d−1 based on the integrated 14C-based gross primary production of 1260mg Cm−2d−1 and the net community production of 626mg Cm−2d−1 in the euphotic zone of the region.

Developing a Great Lakes remote sensing community

Observational data collection of the Laurentian Great Lakes has advanced during the past decade to such a level as to allow real-time analysis from moorings and near real-time from satellite data. Ocean color satellite-based remote sensing provides a rich data set that when properly analyzed allows for the generation of geospatial maps of chlorophyll, dissolved organic carbon, suspended minerals, harmful algae blooms (HABs), surface plumes, benthic vegetation communities, primary productivity (pp) and optical water properties (extinction coefficient (kd), photosynthetically active radiation (PAR) and photic zone depth) on a 2 m to 1 km grid dating back in some cases to the early 1970s. Microwave satellite sensors such as synthetic aperture radar (SAR) and scatterometers provide near real-time information on lake ice cover, winds and waves. Multi-temporal Landsat and ALOS PALSAR satellite data are also being used in the Great Lakes to map wetlands and invasive plantswithin these coastal areas. Airborne LiDARs alsoprovide useful nearshore water depth and bottom type mapping in Great Lakes waters. Making the most of such improvements in the historical Great Lakes datasetwill require diligence and a comprehensive strategy, with recognition of the importance of open collaboration in developing a regional working strategy for remote sensing technologies, sensor data applications, and the datamanagementmethods thatwill integrate the technologies within regional and global observation systems. The IOOS/Great Lakes Observing System (GLOS), in collaboration with NOAA/GLERL, and in conjunction with the ongoing NOAA CoastWatch Great Lakes programwith its stakeholders that include federal, state, local government alongwith theprivate sector and academia, can implement into operational scenarios the suite of remote sensing algorithms developed to generate meaningful Great Lakes products.

Reevaluating the Generality of an Empirical Model for Light-Limited Primary Production in the San Francisco Estuary

Depth-integrated primary production (ΣP, in grams of carbon per square meter per day) was measured using 14C in the northern San Francisco Estuary (SFE) from March through August of 2006 and 2007. Determinations of ΣP were then used to calibrate a published light-utilization model that relates ΣP to a composite parameter of chlorophyll, solar irradiance, and photic zone depth. The resultant calibration coefficient, Ψ, varied by a factor of nearly two between 2006 and 2007 and was lower than determined in previous calibrations for the estuary. The now chronically low chlorophyll concentrations in the SFE have resulted in lower predictive power of the light-utilization model. The variation in Ψ was likely the result of interannual variation in phytoplankton assimilation number. These results suggest that using a single Ψ may yield large errors in estimated estuarine production when applied overbroad spatial and temporal scales. Given the food-limited condition of the SFE, it appears that direct measurements of primary production are necessary for accurately characterizing the base of the estuarine food web.

Vertical transport of particulate organic matter regulated by fjord topography

The deepwater in many areas with restricted water exchange suffers from low oxygen concentrations due to degradation of organic matter. The objective of this study was to investigate how topography may influence the vertical transport, Fc, of particulate organic matter, POM, to the deepwater. A conceptual/analytical model for Fc was developed covering the combination of the three possible sources of POM/nutrients to an enclosed area; the coastal water, local supply, and nutrient rich local deepwater. The mathematical formulation of the conceptual model includes several factors describing to which degree various physical mechanisms in the fjord are influencing Fc. The model consists of submodels for the different sources of POM/nutrients. A one‐dimensional process oriented numerical model was used to test the conceptual model. Restricted water exchange with the coastal water led to decreased import of POM and thereby decreased vertical transport, Fc, of coastal POM. The contribution to Fc by local input of nutrients to the surface layer was described by a function of the residence time of the water above sill level, Tsw and the time Tp it takes for POM produced in the surface layer to settle below sill level. The recirculation of POM produced due to basin water renewals was shown to be a function of several factors: The relation between the depth of photic zone, Hp and the sill depth, Ht, the vertical velocity of the rising nutrient rich water mass versus the settling velocity of POM, etc. The results based on the conceptual submodels agreed well with the results from the process oriented numerical model. Methods to identify the trophic state of coastal waters, and also simple models to calculate the effect of a local point source, can be found in literature. However, using the model developed in this paper the effects of nutrient enrichment from different sources can be quantified in a simple and more efficient way than earlier. The model can thus be used to categorise areas with respect to sensitivity to local and large scale eutrophication, which is an important step towards better management and more sustainable exploitation of the coastal zone.

Ecosystem metabolism in a dryland river waterhole

Little is known about ecosystem processes in dryland rivers, despite the global distribution of these systems. Those in Australia are characterised by long periods of no flow in which they persist for many months as series of isolated, often turbid, waterholes. We assessed benthic and pelagic primary production, respiration, and bacterial production in one of these waterholes to determine the metabolic balance of the waterhole and resolve the relative importance of autochthonous and allochthonous sources of organic carbon. Despite a photic zone depth of only 0.25 m, three lines of evidence suggested that autochthonous sources of organic carbon were important for fuelling bacterial production under no-flow conditions: the metabolic balance of the waterhole was not indicative of large allochthonous inputs; rates of gross primary production were great enough to meet a substantial fraction of estimated bacterial carbon demand; and pathways for allochthonous carbon to enter the waterhole were limited. These results suggest that models of lake metabolism based on temperate ecosystems can be expanded to include dryland river waterholes, which group with eutrophic lakes owing to their high levels of inorganic nutrients, low allochthonous inputs and autotrophic metabolic balance.

Primary productivity in receiving reservoirs: links to influent streams

Primary productivity and chlorophyll a concentrations were measured in 8 reservoirs in New York City drinking-water-supply watersheds. The light-and-dark bottle O2-change procedure was used to measure gross primary productivity (GPP) once each summer from 2000 to 2002. GPP normalized for photosynthetically active radiation (PAR) in the Neversink and Schoharie averaged only 0.025 and 0.035 g O2/mol quanta, respectively. Values for New Croton and Cannonsville averaged 0.118 and 0.125 g O2/mol quanta, respectively. Values in the other reservoirs (west basin Ashokan, Pepacton, Rondout, and Kensico) were intermediate. Chlorophyll a concentrations in reservoir photic zones ranged from mean values of <10 to 100 mg/m2, with highest values in New Croton and Cannonsville and lowest concentrations in Neversink, Pepacton, and Schoharie. Cannonsville was eutrophic, and New Croton was at the mesotrophic–eutrophic boundary. Neversink, Schoharie, and Pepacton were at the oligotrophic–mesotrophic boundary, and the remaining reservoirs (Kensico, Rondout, and west basin Ashokan) were mesotrophic. Reservoir conditions were related to watershed-scale land use. Gradients within reservoirs in chlorophyll a, depth of photic zone, and primary productivity indicated an influence of the major tributary on reservoir conditions in several of the reservoirs.

Assessing Secchi and photic zone depth in the Baltic Sea from satellite data.

Long-term trends in the Secchi depth of the Baltic Sea have been interpreted in terms of eutrophication. The spectral attenuation coefficient Kd (490) can be estimated from remote sensing data. Given the empirical and theoretical relationships between diffuse attenuation and Secchi depth, it is therefore possible to estimate the trophic state from remote sensing data. This paper considers relationships among remotely sensed and in-water measured K (490), and Secchi depth data obtained during dedicated sea-truthing campaigns in the eastern Baltic Proper in 1999 (4) and in the western Baltic Proper/Himmerfjärden area during 2001 and 2002. In-water measurements are used to establish the relationship between the PAR and the spectral attenuation coefficient in the Baltic Sea via regression analysis. The analysis showed that in the area of investigation Kd(490) is about 1.48 times higher than Kd (PAR). This relationship is then used to define the link between the photic zone depth and the remote sensing optical depth, Kd (490)-1. The results show that the depth of the euphotic zone is about 6.8 times Kd (490)-1. The regression analysis between Kd (PAR) and Secchi depth confirmed previous work that Kd (PAR) is about 1.7 of the inverse Secchi depth. Furthermore, an in-water algorithm between Secchi depth and Kd (490) is used to simulate a Secchi depth map of the Baltic Sea from SeaWiFS Kd(490) data. This map is verified against sea-truthing data. Kd (490) data derived from satellite is compared to in situ Kd (490), and the sources of error are discussed.

Assessing Secchi and Photic Zone Depth in the Baltic Sea from Satellite Data

Assessing Secchi and Photic Zone Depth in the Baltic Sea from Satellite Data

Comparison of thermal stratification, light attenuation, and chlorophyll-a dynamics between the ends of Lake Tanganyika

Thermal structure, light attenuation, and chlorophyll-a dynamics within the upper 100 m at the northern and southern ends of Lake Tanganyika were measured from August 1995–July 1996. Pronounced variability of thermal structure in time, depth and region were observed. During the dry windy season (June–September), the water column in the south was largely isothermal down to 100 m, while in the north stratification was detectable. Subsequently, in October, cessation of winds coincided with a reestablishment and strengthening of thermal stratification in the south, and a distinct rise of a weakened thermocline in the north was accompanied by an increase in epilimnetic concentrations of dissolved solids. The depth of photic zone was highly variable with the average position of 1% of incident photosynthetically active radiance found between 20 and 70 m, sometimes showing vertical shifts of 25 m a week. Weekly observations showed that concentrations of chlorophyll-a were rather similar at both ends of the lake. At both stations, periods of unstable or absent thermal stratification were accompanied by increased concentrations of chlorophyll-a, although in the north chlorophyll-a also peaked during periods of deep thermal stratification and low rates of mixing. Concentrations of chlorophyll-a did not show significant relationship with the available amount of underwater available photosynthetically active radiance (Im from 11–173 μEm−2 s−). We suggest that in Lake Tanganyika chlorophyll-a maxima can occur whenever growth rates exceed spatial mixing rates, that is, if mixing is less than a critical turbulence. Our observations underscore the consequences of hydrodynamic processes on biological productivity either by securing internal nutrients or by controlling the timing and magnitude of phytoplankton biomass production.

Algal respiration and the regulation of phytoplankton biomass in a polymictic tropical lake (Lake Xolotlán, Nicaragua)

Community respiration in tropical Lake Xolotlan, Nicaragua, was assessed seasonally and during diurnal cycles, via oxygen consumption in bottle enclosures. Results were analysed in relation to phytoplankton biomass, mixing depth, depth of photic zone and phytoplankton production. A great part of community respiration was associated with the heterotrophic activity of the phytoplankton biomass or its degradation by bacteria and 80% of the variability in oxygen consumption was explained by the variation of chlorophyll-a. Specific rate of respiration was 1.5 mg O2 mg Chla-1 h-1 during diurnal cycles, which corresponded to less than 5% of the specific rate at optimum depth of production. Still, diurnal water column respiratory losses were always of the same magnitude as the total photosynthetic gains in the photic zone, since the mixing depth exceeded the depth of the photic zone. Total column net growth was zero at a ratio between depth of photic zone and mixing depth of 0.19. Water level variations however altered the mixing depth and affected this ratio and net growth. As a consequence, the phytoplankton biomass either increased or decreased until the ratio was re-established through changes of the photic zone depth, which was governed by the phytoplankton biomass itself through the chlorophyll-a light attenuation.

Gaining insight into the seasonal variability of CO 2 at ocean station P using an upper ocean model

An eddy-kinetic energy model for simulations of the oceanic vertical mixing ( Gaspar et al., 1990, Journal of Geophysical Research, 95, 16179–16193) is used to investigate the seasonal cycle of dissolved total CO 2 and CO 2 partial pressure at Station P (Gulf of Alaska) during the years 1971 and 1972. The model simulates relatively weak seasonal variations of surface seawater pCO 2. All processes appear to contribute to the same order of magnitude: interaction of gas exchange and biology tends to compensate for the thermodynamical effect on pCO 2. The influence of various model parameters (total annual production of O 2, P, depth of photic zone, z 0, gas transfer velocity, K) on the seasonal variability of seawater pCO 2 and air-sea CO 2 flux is examined. Total annual production, P, seems to be the most influential. The model also suggests a strong short-term variability of pCO 2 during episodic events. Model results show that the area around Station P behaves as a weak or neutral CO 2 source during summer and as a net sink during other seasons. The net annual CO 2 flux is an invasion flux, with values of 0.88 and 0.70 mol m −2 y −1 for 1971 and 1972, respectively, in agreement with the flux of 0.7 mol m −2 y −1 derived from observations during 1973–1978 ( Wong and Chan, 1991, Tellus, 43B, 206–223).

Modelling the seasonal cycle of dissolved oxygen in the upper ocean at ocean weather station P

Three main processes regulate the variations of dissolved O 2 concentrations in the surface waters: gas exchange at the air-sea interface, vertical mixing and biological activity of marine organisms. A one-dimensional integral mixed layer model ( Gaspar, 1988) is used to study the temporal evolution of monthly averaged dissolved O 2 content of surface waters at Ocean Weather Station P, and to assess the relative importance of the various contributing mechanisms during 1969–1972. Production and consumption due to biological activity are taken into account as an input function of the model. A large part of the seasonal signal of dissolved O 2 in surface waters can be reproduced by the physical model without biological activity. However, kinetics of gas exchange, biological production and entrainment of sub-mixed layer water all contribute by the same order of magnitude to supersaturation during warming periods and undersaturation during cooling periods. Various shapes (over depth and time) of production-consumption function have been tested for the year 1970. Most of the evolution of monthly average dissolved O 2 in the surface waters can be obtained (1) with a total annual production rate of the order of 5 mol O 2 m −2 y −1, (2) with a constant production throughout the year and in the 0–50 m layer, and (3) with logarithmic decrease in consumption between 50 and 300 m. The relative influence of various parameters on the three processes supplying O 2 to the surface waters is investigated. Total annual production P seems to be the most influential. Vertical mixing and depth of photic zone, z 0, affect the gas exchange flux during the cooling season. Episodic events, like storms, modify the supersaturation of the mixed layer O 2 content by up to 4 mmol m −3, but gas exchange later draws back this content towards a smooth evolution curve. Finally, the sensitivity of the net annual gas exchange to various parameters is too large for the model to provide a reliable value.

Phytoplankton production in two east coast estuaries: Photosynthesis-light functions and patterns of carbon assimilation in Chesapeake and Delaware Bays

Chesapeake Bay is a large and productive estuary that has received close scrutiny in recent years because of indications that its water quality and biota have been damaged by man's activities. Data on primary production for the estuary as a whole, however, are surprisingly sparse. We describe here the distribution of photosynthetic carbon assimilation by phytoplankton in Chesapeake Bay, and relate productivity patterns to hydrographic characteristics of the estuary. Between March 1982 and April 1983, a series of four cruises was conducted on Chesapeake Bay, and two cruises on the urbanized Delaware Bay for comparison. The upper Chesapeake and Delaware were highly turbid with high concentrations of suspended particulate matter and dissolved inorganic nutrients. Low chlorophyll concentrations were usually found in these areas of high turbidity, despite the abundance of nutrients, suggesting light limitation. Application of Wofsy's (1983) model of phytoplanton growth confirmed this suggestion. Chlorophyll and productivity maxima usually occurred seaward of the turbidity maxima where light penetration increased and suffient nutrients were present to support active phytoplankton growth. Further seaward of the chlorophyll maxima in the Chesapeake, the photic zone depth increased, concentrations of nutrients decreased, and phytoplankton biomass decreased, suggesting that nutrient availability, rather than light, controlled phytoplankton growth in the lower portion of the estuary. In contrast to the Chesapeake, Delaware Bay was more turbid, had generally higher nutrient concentrations, and was lower in phytoplankton productivity. The chlorophyll maxima and region of rapid phytoplankton growth occurred further toward the lower estuary and shelf regions in Delaware Bay because the high turbidity extended further seaward. Nutrients were never depleted at the shelf end of the estuary sufficiently to retard phytoplankton growth. Photosynthesis-irradiance (P-I) curves from simulated in situ and constant intensity incubations showed a strong correlation of the light-limited slope ( a B) with the light-saturated rate ( P m B ) on each cruise. Spatial variations in P m B corresponded to patterns of phytoplankton abundance, as did integral production (PP) and carbon-based growth rates ( μ C, μ m), and photosynthetic parameters varied significantly with temperature.

Phytoplankton associations with the variable nepheloid layer on the Texas continental shelf

Four 24-h cruises occupied a station at 33m depth on the Texas continental shelf during June, July, September and November 1978. Samples collected at 4-h intervals investigated the state of selected physical, chemical and biological factors potentially important to phytoplankton associated with the nepheloid layer. The nepheloid layer varied in thickness and in density within and between cruises. Phytoplankton biomass was differentially associated with the nepheloid layer in June and July but not in September and November. The first two cruises showed a mixed phytoplankton community (>20μm) composed of pelagic and benthic species and a net carbon-fixation within the nepheloid layer. Based on photic zone depth, enhanced phytoplankton biomass resulting from growth can theoretically accumulate within the nepheloid layer between the coast and the 70-m contour from July to September. This phytoplankton concentration may provide an important food reservoir to eligible herbivores during the summer months.

Water Transparency of Lake Superior in 1973

Measurements relating to water transparency were made at roughly monthly intervals during the period May through November 1973 on Lake Superior. Spectrometer data augmented with extensive transmissometer and Secchi disc observations at a grid of stations over the lake were used to compute mean vertical extinction coefficients and photic depth for photosynthetically active radiation in the wavelength band 400-500 nm. These permitted description of the temporal and spatial variability of photic conditions throughout the major photosynthetic production period. The data show spatial and temporal variability in water transparency between areas and, for some zones, considerable departures from the lakewide mean are evident. The mean vertical extinction ranges from approximately 0.15 to 0.25 m −1 for the majority of zones and relatively good correspondence is observed with seasonal changes in chlorophyll a concentration. Considerably lower rates of light attenuation occur in the deeper midlake zones. Photic zone depth in these areas characteristically range from 20 to 30 meters. Reduced water transparency generally predominates along coastal areas in the proximity of Duluth, Thunder Bay, the southwest red clay portions of the lake, Apostle Is., Nipigon-Black Bay, Marathon, and Whitefish Bay. Photic zone depth in these areas are primarily less than 20 metres throughout the photosynthetic production period. In the vicinity of Duluth, mean photic depth values of less than 10 metres are observed with extreme minimums approaching 2 metres.