Secchi Depth Measurements Research Articles

Spatial and temporal variability of water transparency, its primary drivers, and other optical properties in the Hudson River Estuary

Water transparency, measured as the diffuse attenuation coefficient of photosynthetically-active (400–700 nm) radiation, KdPAR, was measured monthly from May–Oct 2020 at 73 locations along 250 km of the Hudson River estuary (HRE) between New York City, NY and Waterford, NY. Three primary optical drivers of KdPAR variability: chlorophyll concentration (CHL), turbidity, and light absorption by colored dissolved organic matter (aCDOM), were measured concurrently to assess their relative importance to spatial and temporal KdPAR patterns. KdPAR was higher and more variable in the brackish HRE relative to limnetic regions further north. KdPAR was also significantly higher nearshore and near sewage treatment plant outfalls relative to midchannel sites. Temporal variability at individual stations was generally lower than cross-channel or cross-region spatial variability. Turbidity was the most important factor in KdPAR variability in all regions of the HRE, followed by CHL, with aCDOM playing a lesser role. Although many water quality variables have changed in the HRE over recent decades, KdPAR measurements in the brackish regions overlapped with published observations from the 1970s–1990s. KdPAR in the limnetic regions was lower in 2020 than measurements made prior to the Zebra mussel invasion in the early 1990s, but similar to measurements in the limnetic regions made since the invasion. Results of this study can be used to optimize satellite-based remote sensing algorithms for synoptic water quality observations of the HRE. Proposed future alterations in the HRE, including dredging and the construction of storm-surge barriers, would likely change existing KdPAR patterns. This study therefore provides an updated baseline for optical parameters in the HRE and highlights the importance of understanding the impacts of any proposed alterations to upper-water-column turbidity in this system. Results also demonstrate that KdPAR, euphotic depths, and Secchi depth measurements were positively correlated throughout the HRE. Secchi depth measurements may therefore be useful for additional retrospective studies, and future monitoring (including by satellite-derived KdPAR) of water transparency in the HRE.

Modeling Coastal Water Clarity Using Landsat‐8 and Sentinel‐2

AbstractUnderstanding and attributing changes to water quality is essential to the study and management of coastal ecosystems and the ecological functions they sustain (e.g., primary productivity, predation, and submerged aquatic vegetation growth). However, describing patterns of water clarity—a key aspect of water quality—over meaningful scales in space and time is challenged by high spatial and temporal variability due to natural and anthropogenic processes. Regionally tuned satellite algorithms can provide a more complete understanding of coastal water clarity changes and drivers. In this study, we used open‐access satellite data and low‐cost in situ methods to improve estimates of water clarity in an optically complex coastal water body. Specifically, we created a remote sensing water clarity product by compiling Landsat‐8 and Sentinel‐2 reflectance data with long‐term Secchi depth measurements at 12 sites over 8 years in a shallow turbid coastal lagoon system in Virginia, USA. Our satellite‐based model explained ∼33% of the variation in in situ water clarity. Our approach increases the spatiotemporal coverage of in situ water clarity data and improves estimates from bio‐optical algorithms that overpredicted water clarity. This could lead to a better understanding of water clarity changes and drivers to better predict how water quality will change in the future.

Citizen Science Tools Reveal Changes in Estuarine Water Quality Following Demolition of Buildings

Turbidity and water colour are two easily measurable properties used to monitor pollution. Here, we highlight the utility of a low-cost device—3D printed, hand-held Mini Secchi disk (3DMSD) with Forel-Ule (FU) colour scale sticker on its outer casing—in combination with a mobile phone application (‘TurbAqua’) that was provided to laymen for assessing the water quality of a shallow lake region after demolition of four high-rise buildings on the shores of the lake. The demolition of the buildings in January 2020 on the banks of a tropical estuary—Vembanad Lake (a Ramsar site) in southern India—for violation of Indian Coastal Regulation Zone norms created public uproar, owing to the consequences of subsequent air and water pollution. Measurements of Secchi depth and water colour using the 3DMSD along with measurements of other important water quality variables such as temperature, salinity, pH, and dissolved oxygen (DO) using portable instruments were taken for a duration of five weeks after the demolition to assess the changes in water quality. Paired t-test analyses of variations in water quality variables between the second week of demolition and consecutive weeks up to the fifth week showed that there were significant increases in pH, dissolved oxygen, and Secchi depth over time, i.e., the impact of demolition waste on the Vembanad Lake water quality was found to be relatively short-lived, with water clarity, colour, and DO returning to levels typical of that period of year within 4–5 weeks. With increasing duration after demolition, there was a general decrease in the FU colour index to 17 at most stations, but it did not drop to 15 or below, i.e., towards green or blue colour indicating clearer waters, during the sampling period. There was no significant change in salinity from the second week to the fifth week after demolition, suggesting little influence of other factors (e.g., precipitation or changes in tidal currents) on the inferred impact of demolition waste. Comparison with pre-demolition conditions in the previous year (2019) showed that the relative changes in DO, Secchi depth, and pH were very high in 2020, clearly depicting the impact of demolition waste on the water quality of the lake. Match-ups of the turbidity of the water column immediately before and after the demolition using Sentinel 2 data were in good agreement with the in situ data collected. Our study highlights the power of citizen science tools in monitoring lakes and managing water resources and articulates how these activities provide support to Sustainable Development Goal (SDG) targets on Health (Goal 3), Water quality (Goal 6), and Life under the water (Goal 14).

Effects of lake restoration on breeding abundance of globally declining common pochard (Aythya ferina L.)

Common pochard Aythya ferina are rapidly declining globally, partly due to water quality change at breeding habitats. Lake restoration at two southern Danish lakes (external nutrient loading reduction, bream Abramis brama and roach Rutilus rutilus removal and stocking of pike Esox lucius) improved water clarity and submerged macrophyte cover. Nesting pochard on one lake increased from 2.3 females per annum pre-treatment to 13.9 afterwards and from 22.7 females to 99.6 post-treatment at a second. Numbers fell from 27.7 to 11.3 at a third untreated lake with consistently high water clarity, but which was colonised and became dominated by holly-leaved naiad Najas marina, which provides no food resources for breeding pochard. Linear modelling (controlling for serial autocorrelation) showed statistically significant effects of annual summer measurements of suspended solids (negative) and Secchi depth (positive) on pochard abundance at both restored lakes and chlorophyll (negative) at one of those lakes, but no effects at the third. Breeding pochard numbers also correlated positively with submerged macrophyte cover at one restored lake with adequate data. Results support the hypothesis that lake restoration to improve water quality enhances conditions for locally breeding pochard, as long as restored conditions secure appropriate submerged macrophyte communities for nesting pochard.

Quantifying seagrass light requirements using an algorithm to spatially resolve depth of colonization.

Depth of colonization (Zc ) is a useful seagrass growth metric that describes seagrass response to light availability. Similarly, percent surface irradiance at Zc (% SI) is an indicator of seagrass light requirements with applications in seagrass ecology and management. Methods for estimating Zc and % SI are highly variable making meaningful comparisons difficult. A new algorithm is presented to compute maps of median and maximum Zc , Zc, med and Zc,max , respectively, for four Florida coastal areas (Big Bend, Tampa Bay, Choctawhatchee Bay, Indian River Lagoon). Maps of light attenuation (Kd ) based on MODIS satellite imagery, PAR profiles, and Secchi depth measurements were combined with seagrass growth estimates to produce maps of % SI at Zc,med and Zc,max . Among estuary segments, mean Zc,med varied from (±s.e.) 0.80±0.13 m for Old Tampa Bay to 2.33±0.26 m for Western Choctawhatchee Bay. Standard errors for Zc,med were 1-10% of the segment means. Percent SI at Zc,med averaged 18% for Indian River Lagoon (range = 9-24%), 42% for Tampa Bay (37-48%) and 58% for Choctawhatchee Bay (51-75%). Estimates of % SI were significantly lower in Indian River Lagoon than in the other estuaries, while estimates for Tampa Bay and Choctawhatchee Bay were higher than the often cited estimate of 20%. Spatial gradients in depth of colonization and % SI were apparent in all estuaries. The analytical approach could be applied easily to new data from these estuaries or to other estuaries and could be incorporated routinely in assessments of seagrass status and condition.

The Pressure of Society on Water Quality: A Land Use Impact Study of Lake Ripley in Oakland, Wisconsin

Eutrophication of lakes occurs naturally over time, but the eutrophication rate can be accelerated by human activities. Agriculture land use can negatively impact water quality of lakes due to nutrient pollution. This research investigates the impacts of agricultural land use on the water quality of Lake Ripley in Oakland, Wisconsin from 1993 to 2011. This study performs a regression analysis which incorporates four years of National Land Cover Database (NLCD) data, eight spatial categories based on hydrological flow length across topographic surface, and a weighting technique to calculate land use percentages. The results indicate that the combination of agricultural land use and rainfall variables are significantly related to chlorophyll a and total phosphorus concentrations, while these variables do not appear to affect Secchi depth measurements. Due to the near flat topography of the Lake Ripley watershed, agricultural land use within the two spatial regions closest to Lake Ripley and its inlet stream had the largest impact on Lake Ripley's water quality.

Seasonal dynamics of dissolved, particulate and microbial components of a tidal saltmarsh-dominated estuary under contrasting levels of freshwater discharge

Tidal Spartina-dominated saltmarshes and estuaries on the Southeast US coast are global hotspots of productivity. In coastal Georgia, tidal amplitudes and saltmarsh productivity are the highest along the Southeast US coast. Coastal Georgia is characterized by a humid subtropical seasonal climate, and inter-annual variability in precipitation, and freshwater discharge. The 2012–2013 timeframe encompassed contrasting levels of discharge for the Savannah River, a major Georgia river, with a 4.3-fold greater discharge in summer 2013 relative to summer 2012. In situ measurements of temperature, salinity, precipitation and Secchi depth, and water samples were collected weekly at high tide throughout 2012 and 2013 from the Skidaway River Estuary, a tidal saltmarsh-dominated estuary in coastal Georgia influenced by Savannah River hydrology. The effects of elevated discharge on the seasonal trends of water column components were evaluated. The shift from low discharge (2012) to high discharge (2013) led to decreased salinity in summer 2013, but no significant increases in inorganic nutrient (NH4, NOx, SiO2 and PO4) concentrations. Dissolved inorganic carbon (DIC) concentrations decreased, and DIC stable isotopic signatures (δ13C-DIC values) were depleted in summer 2013 relative to summer 2012. In 2013 dissolved organic carbon (DOC) concentrations, chromophoric and fluorescent dissolved organic matter (DOM: CDOM, FDOM) intensities, specific UV-absorbance (SUVA254) and relative humic-like fluorescence were all higher than in 2012, indicating that, as discharge increased in 2013, estuarine water became enriched in terrigenous DOM. Secchi depth and particulate organic carbon (POC) and nitrogen (PON) concentrations displayed clear seasonal patterns that were not significantly altered by discharge. However, δ13C-POC and δ15N-PON isotopic signatures indicated higher terrigenous contributions at elevated discharge. Discharge influenced cyanobacterial composition, but did not affect total abundance of phytoplankton (<52 μm) or chlorophyll-a concentrations, a proxy for phytoplankton biomass. Phytoplankton community dynamics were primarily seasonally-driven. Bacterioplankton abundance and community composition, based upon flow cytometry, were affected by discharge, possibly due to decreased salinity and/or increased inputs of terrigenous DOM. Seasonal patterns in inorganic nutrient, POC, PON and chlorophyll-a concentrations, and Secchi depth were not significantly influenced by the 2013 increase in discharge. For other components, most notably δ13C-DIC values, DOM and bacterioplankton, the influence of increased discharge in 2013 was superimposed upon their seasonal patterns. This study showed that in addition to tidal mixing and in situ saltmarsh and estuarine production and removal processes, the level of riverine freshwater discharge impacted the quantity and character of many water column components in this tidal saltmarsh ecosystem.

Assessing ecological water quality of freshwaters: PhyCoI—a new phytoplankton community Index

We propose and test a new Phytoplankton Community Index (PhyCoI) for monitoring the ecological status of lakes and reservoirs. The design of our PhyCoI is based on the fact that phytoplankton biomass and community structure respond to changes in water quality (mainly eutrophication) and by themselves also influence water quality. In order to accommodate this double role of phytoplankton as indicator and impact, PhyCoI is based on phytoplankton community properties at different hierarchical levels combining both specific metrics (total biomass, taxonomic group biomass, cyanobacteria contribution, taxonomic group species richness) and new or modified sub-indices. It is calculated from the scores of the different metrics/sub-indices resulting in a final index value in the range from 0 to 5, to assess water quality on the basis of five ecological classes according to the Water Framework Directive (WFD). The test of PhyCoI was based on Carlson's Trophic State Index (TSISD) based on water transparency (Secchi depth) in 26 Greek lakes and reservoirs covering the entire spectrum from oligotrophic to hypertrophic. A highly significant relationship at p<0.001 between the two variables was found, with the values of the PhyCoI declining with increasing TSISD. Furthermore, a significant relationship between the PhyCoI and land use types at the watershed of the studied freshwaters was found identifying permanent crops, pastures and shrubs and herbaceous vegetation associations as significant predictors of PhyCoI values. Because of the amount of labor involved in obtaining the PhyCoI we suggest to combine low frequency PhyCoI determinations with a high frequency Secchi depth measurements for practical monitoring purposes.

Biomass-Cover Relationship for Eelgrass Meadows

Eelgrass meadows play key roles in coastal ecosystems, and the extent of the standing biomass is focal to address ecosystem functioning. Eelgrass cover is commonly assessed in marine monitoring programs while biomass sampling is destructive and expensive. Therefore, we have proposed a functional relationship that translates eelgrass cover into aboveground biomass using site-specific information on Secchi depth or light attenuation. The relationship was estimated by non-linear regression on 791 combined observations of eelgrass cover and biomass from eight different coastal sites in Denmark. Eelgrass biomass initially increased with cover and flattened out as cover exceeded 40–50 % due to increased self-shading. Decreasing light energy with depth reduced the eelgrass biomass potential (assessed at 100 % cover), and this reduction was stronger for coastal sites with lower water transparency. Moreover, the biomass potential varied seasonally from around 110–140 g DW m−2 in spring months to a peak of 241 g DW m−2 in August, consistent with other seasonal studies. The model explained 56 % of the variation in log-transformed biomasses, but significant variation between coastal sites still remained, deviating between −23 and 39 % from the mean relationship. These site-specific deviations could be due to differences in losses related to grazing, drifting algae and epiphytes, better light capture by dense canopies, as well as differences in how well light conditions within eelgrass meadows are represented by actual measurements of Secchi depth and light attenuation. The relationship can be employed to estimate eelgrass biomass of entire coastal ecosystems from observations of eelgrass cover and depth.

A method for estimating the diffuse attenuation coefficient (KdPAR) from paired temperature sensors

AbstractA new method for estimating the diffuse attenuation coefficient for photosynthetically active radiation (KdPAR) from paired temperature sensors was derived. We show that during cases where the attenuation of penetrating shortwave solar radiation is the dominant source of temperature changes, time series measurements of water temperatures at multiple depths (z1 and z2) are related to one another by a linear scaling factor (α). KdPAR can then be estimated by the simple equation KdPAR = ln(α)/(z2−z1). A suggested workflow is presented that outlines procedures for calculating KdPAR according to this paired temperature sensor (PTS) method. This method is best suited for conditions when radiative temperature gains are large relative to physical noise. These conditions occur frequently on water bodies with low wind and/or high KdPARs but can be used for other types of lakes during time periods of low wind and/or where spatially redundant measurements of temperatures are available. The optimal vertical placement of temperature sensors according to a priori knowledge of KdPAR is also described. This information can be used to inform the design of future sensor deployments using the PTS method or for campaigns where characterizing sub‐daily changes in temperatures is important. The PTS method provides a novel method to characterize light attenuation in aquatic ecosystems without expensive radiometric equipment or the user subjectivity inherent in Secchi depth measurements. This method also can enable the estimation of KdPAR at higher frequencies than many manual monitoring programs allow.

HYDROPERIOD CLASSIFICATION OF CERVANTES COOLIMBA COASTAL WETLANDS USING LANDSAT TIME SERIES IMAGERY

Abstract. Geomorphic classification of wetlands relies on information regarding landform and hydroperiod. Additional attributes of wetland size, vegetation cover and salinity can be collected for lower order classification. Hydroperiod is important in determining features that characterize the ecological function of the wetland. This study examines how Landsat time series imagery was used to describe the hydroperiod of wetlands along the coastal zone between Cervantes to Coolimba in Western Australia. Inundation extent maps were derived using 17, Landsat Band 5 images captured between 1988 and 2011. The number of times a wetland basin was more than 10 % inundated was then calculated. This inundation frequency dataset was then the basis for hydroperiod classification. Wetlands were classified as being permanently inundated if they had more than 10% inundation in 12 or more of the 17 images available (approximately 70%). A Landsat image captured 2 weeks prior to field work was used to determine Band 5 thresholds to identify areas of inundation and assess classification accuracy. Field measurements of surface water cover, depth, secchi depth, vegetation cover and salinity were taken across 16 plots located close to inundation boundaries. The accuracy of the inundation maps was highly dependent on the degree of vegetation cover. Mapping accuracy was heavily influenced by vegetation cover and achieved an overall accuracy of 87.5%. The resulting hydroperiod datasets provides an accurate record of inundation frequency which can be used to aid classification of wetlands and also allows changes to inundation frequency over time to be assessed.

Estimate of water inherent optical properties from Secchi depth

This paper suggests a new version of the Secchi disk theory which shows a connection between Secchi depth measurements and inherent optical properties (IOP) of water such as the extinction coefficient, the single scattering albedo, and the backscattering coefficient. Ways around Preisendorfer’s objection to using measurements of the Secchi depth for determining the IOP are proposed. This theory is compared with a marine experiment and its accuracy under different conditions is estimated.

Optical remote sensing of turbidity and total suspended matter in the Gulf of Gabes

Optical remote sensing was used to provide scientific information to support environmental management in the Gulf of Gabes that is located in the southeastern coast of Tunisia. This region is characterized by shallow continental shelf subjected to semi-diurnal tides. Industrial activities in this area since the early 1970s may have contributed to the degradation of the biodiversity of the ecosystem with eutrophication problems and disappearance of benthic and planktonic species. To assess the long-term effect of anthropogenic and natural discharges on the Gulf of Gabes, the optical environment of the coastal waters is assessed from in situ measurements of total suspended matter concentration (TSM), Secchi depth and turbidity (TU). This monitoring requires regular seaborne measurements (monthly), which is very expensive and difficult to obtain. The objective of the present study is the evaluation of the Moderate Resolution Imaging Spectrometer (MODIS) AQUA data compared with two sampling campaigns realized at the study area. To map turbidity data from MODIS images, a semi-empirical algorithm was applied at band 667 nm. This bio-optical algorithm has already been calibrated and validated on the Belgian coast. The validation of this algorithm on the Gulf of Gabes using in situ measurements of turbidity and remotely sensed turbidity obtained from MODIS imagery shows a correlation coefficient of 68.9%. Seasonal and annual average maps for TSM and TU were then computed over the Gulf of Gabes using MODIS imagery. The obtained results of TSM and TU from remotely sensed data are conformable with those obtained through the analysis of in situ measurements. Therefore, remote sensing techniques offer a better and efficient tool for mapping and monitoring turbidity over the whole region.

Remote Sensing for Water Quality Monitoring &amp; Watershed Assessment On the Lake Traverse Reservation

AbstractDuring the summer of 2010, three student interns from Sisseton Wahpeton Tribal College along with researchers from South Dakota State University engaged in a pilot project to assess the feasibility of using Landsat imagery to determine water transparency in reservation lakes in northeastern South Dakota. The students were trained to assist with data collection, image processing, and spatial and statistical analyses. Secchi depth measurements and lake algae bloom observations were made on cloud-free Landsat overpass days. Measurement locations were geo-referenced using GPS. Field observations and satellite image classification and interpretation techniques were used to create a land use map for the watershed and explore possible land-use-related causes of the variation in water transparency. The results of this project included a better understanding of the relationship between Landsat images and on site observations of water transparency at first and then land use and water quality in reservation lakes.

On non-Eltonian methods of hunting Cladocera, or impacts of the introduction of planktivorous fish on zooplankton composition and clear-water phase occurrence in a Mediterranean reservoir

Among the topics covered by Hutchinson’s Santa Rosalia article, the question of the shortening and lengthening of food webs occupies a central role. As Hutchinson realized, at the time scales of ecological studies, the impact of invader species on established food webs is the fastest shortcut to the shortening or lengthening of the food webs. The construction of thousands of dams in Spain during the last century has offered ecologists a good opportunity to test the effects of invader fish species on the plankton dynamics of these systems. In this article, a series of data related to the food web structure of Sau Reservoir is analyzed for the period 1997–2005. Parameters such as Secchi depth and chlorophyll concentration, as well as abundance and size structure of zooplankton, have been matched to the zooplankton dynamics in the reservoir. Most of the changes detected within this period are attributed to the introduction of zooplanktivorous fish in the reservoir. The Secchi depth measurements have showed a progressive diminution in the clear-water phase during recent years. These changes have been related to the decrease in the abundance of Daphnia and to the reduction of the size of zooplankton, which help to explain concomitant increases in the chlorophyll concentration in the same period. Other observed changes in the composition of the zooplankton community have been the substitution of Daphnia by Bosmina and the increase in the abundance of rotifers. Thus, the annual average abundance of Bosmina in 1997 was 70% of cladocerans, while in 2005 it reached 98%. In parallel, the percentage occurrence of individual rotifers was 40% of total zooplankton numbers but had risen to 85% at the end of the period. All these changes are attributed to the artificial expansion of the food web through stocking of the reservoir with zooplanktivorous fish (Rutilus rutilus and Alburnus alburnus). This study improves our understanding of the trophic relationships in the food web prior to the introduction of the fish.

Distribution of Virus-Infected Bacteria in the Western Equatorial Pacific

Abstract: Viruses are generally considered an important agent of bacterial loss in diverse marine environments. However, the impact of viruses on bacteria is unknown in the western equatorial Pacific, where surface waters are warm and phytoplankton biomass is low (i.e., oligotrophic). Further, little is known about their importance in the mesopelagial, where bacteria and heterotrophic nanoflagellates are known to be metabolically active. To elucidate the ecological characteristics of viruses in the western equatorial Pacific, abundances of bacteria and viruses were measured, along with frequencies of visibly infected cells (FVIC) and frequencies of dividing cells (FDC) in epipelagic and mesopelagic samples at three stations near the equator from August to September 2002. Measurements of Secchi depth (20 m) and chlorophyll a concentrations (0.07– 0.4 µg chl a liter-1) indicated that the study area was oligotrophic during the investigation. FVIC ranged from 0.4% to 1.8% and 0.5% to 1.8% in the epipelagic an...

Survey of toxic algal (microcystin) distribution in Florida lakes

A survey of microcystin in 187 Florida lakes was completed from Jan to Dec 2006. Annual average microcystin concentrations of the 187 Florida lakes ranged from nondetectable (< 0.1 μ g/L) to 12 μ g/L, with concentrations in individual water samples (N = 862) ranging from nondetectable to 32 μ g/L. Only 7% of all the individual samples exceeded the World Health Organization (WHO) drinking guidance value of 1 μ g/L. Three individual water samples collected from two lakes (0.3%) exceeded the WHO recreational guidance value of 20 μ g/L. Using chlorophyll concentrations and Secchi depth measurements, tables were constructed to predict the probability that microcystin concentrations exceeded the WHO guidance values in Florida lakes. Additionally, six hypereutrophic lakes (Harris Chain of Lakes, Lake County, FL) were studied from Sep 2006 to Aug 2007 and 40% of the samples exceeded 1 μ g/L but none exceeded 20 μ g/L. This study also provides baseline information and relations between trophic status, seasonality, water chemistry and microcystin concentrations in Florida lakes.

Reference conditions for phytoplankton at Danish Water Framework Directive intercalibration sites

Phytoplankton is one of the biological quality elements included in the EU Water Framework Directive (WFD). Classification of water quality according to the WFD is based on the deviation of the present conditions from reference conditions. Given the lack of data from pristine conditions, this study used approximately 100-year-old measurements of Secchi depths from Danish waters in combination with relationships between Secchi depth and chlorophyll a (as a proxy for phytoplankton biomass) obtained from recent monitoring to calculate ‘historical’ or reference chlorophyll a (Chl-a) concentrations. Historical Secchi depth data were available for 9 out of the 11 Danish WFD intercalibration sites. At eight of the sites, reference summer (May-September) Chl-a concentrations were in the range 0.7–1.2 µg l−1. At one site, west of Bornholm in the western Baltic Sea, historical Secchi depth measurements date back to only the late 1950s corresponding to a calculated Chl-a concentration of 1.3 µg l−1. This value cannot be considered representative of reference conditions.

Secchi transparency of Boulder Basin, Lake Mead, Arizona-Nevada: 1990–2007

Over 5,500 individual Secchi readings were taken from Boulder Basin of Lake Mead, Nevada-Arizona, between July 1990 and December 2007. Annual and seasonal patterns are plainly displayed by these data. Variations in Secchi depth measurements are both spatial and temporal. Peak water transparency >15 m generally occurs in April and March and at sampling locations along the thalweg of the Colorado River channel in the Outer portion of Boulder Basin. Transparency steadily decreases toward the tributary inflow into Las Vegas Bay. Transparency in open water of Las Vegas Bay ranges from <1 m during the growing season to 5–7 m in winter. These differences are generally driven by varying algal production which decreases in a cline from Las Vegas Bay to the Outer Basin. Entrainment of small sediment particles from the urban inflow also influences water transparency. Steady lowering of the reservoir during the 2000 to 2007 drought exacerbated the influence of turbidity due to entrainment of former lake bottom sediments into the epilimnion. However, the strongest statistical relationships with decreasing Secchi transparency are with chlorophyll a (chl-a) and total phosphorus (TP). The patterns and relationships presented establish the nature of Boulder Basin's water clarity to 2008 that will be useful in light of imminent and future alterations of the aquatic ecosystem by both anthropogenic and nonanthropogenic factors. Perhaps the largest change will occur due to the recent introduction of the quagga mussel (Dresseina rostriformis bugensis). After discovery in January 2007, the mussel population has been increasing rapidly. The data sets in this presentation form the basis of conditions in Boulder Basin prior to the introduction of quagga mussels.

A 20-year Landsat water clarity census of Minnesota's 10,000 lakes

A 20-year comprehensive water clarity database assembled from Landsat imagery, primarily Thematic Mapper and Enhanced Thematic Mapper Plus, for Minnesota lakes larger than 8 ha in surface area contains data on more than 10,500 lakes at five-year intervals over the period 1985–2005. The reliability of the data was evaluated by examining the precision of repeated measurements on individual lakes within short time periods using data from adjacent overlapping Landsat paths and by comparing water clarity computed from Landsat data to field-collected Secchi depth data. The agreement between satellite data and field measurements of Secchi depth within Landsat paths was strong (average R 2 of 0.83 and range 0.71–0.96). Relationships between late-summer Landsat and field-measured Secchi depth for the combined statewide data similarly were strong ( r 2 of 0.77–0.80 for individual time periods and r 2 = 0.78 for the entire database). Lake clarity has strong geographic patterns in Minnesota; lakes in the south and southwest have low clarity, and lakes in the north and northeast tend to have the highest clarity. This pattern is evident at both the individual lake and the ecoregion level. Mean water clarity in the Northern Lakes and Forest and North Central Hardwood Forest ecoregions in central and northern Minnesota remained stable from 1985 to 2005 while decreasing water clarity trends were detected in the Western Corn Belt Plains and Northern Glaciated Plains ecoregions in southern Minnesota, where agriculture is the predominant land use. Mean water clarity at the statewide level also remained stable with an average around 2.25 m from 1985 to 2005. This assessment demonstrates that satellite imagery can provide an accurate method for obtaining comprehensive spatial and temporal coverage of key water quality characteristics that can be used to detect trends at different geographic scales.