Christopher Strait

Optical characterizations and pursuit of optical closure for the western basin of Lake Erie through in situ measurements

ABSTRACT In situ measurements of inherent (IOPs) and apparent optical properties (AOPs), along with laboratory measurements of optically active constituents, were made at sites (n = 14) in western Lake Erie following a wind event to advance the characterization of the underwater and emergent light fields of these waters and to support related IOP-based model development and testing. Modern instrumentation was used to make spectral (wavelength,&lgr;) measurements of the IOPs of absorption [a(&lgr;)], particulate scattering ]bp(&lgr;)], and particulate backscattering [bbp(&lgr;)] coefficients, and the AOPs of remote sensing reflectance ]Rrs(&lgr;)], and the diffuse attenuation coefficient for downwelling irradiance [Kd(&lgr;)], Optical closure analyses were conducted to demonstrate the credibility of the measurements, by comparing AOP observations to predictions based on radiative transfer expressions that utilized IOP measurements as inputs. Substantial spectral variations in a and its contributing components, and more modest wavelength dependencies for bp and bbp, were documented that are consistent with observations reported for marine case 2 systems. The backscattering ratio, bbp:bp, was strongly positively related to the contribution of minerogenic particles to the overall concentration of suspended particulate material. Major spatial differences in both IOPs and AOPs were observed that were driven by the attendant differences in the concentrations and composition of the optically active constituents, but particularly minerogenic particles, mediated in part by sediment resuspension. Good optical closure between the independently measured IOPs and AOPs was achieved. Direct measurement of bbp(&lgr;) was found to be critical to pursue closure for Rrs(&lgr;) and thereby support related remote sensing initiatives.

Light absorbing components in the Finger Lakes of New York

Light absorbing constituents are important regulators of the underwater light field and the signal available for remote sensing of water quality. The spectral characteristics, magnitude and variability of the components of light absorption are documented and contrasted here for the broad range of conditions manifested in the 11 Finger Lakes of New York. The absorption coefficient, a (m -1 ), was partitioned according to the additive components of colored dissolved organic matter (a CDOM ), non-algal particles (a NAP ), phytoplankton (a Φ ), and water itself (a w ; known), using laboratory spectrophotometric protocols on near surface samples collected monthly in 2004. Bulk in-situ measurements included Secchi disc depth (SD), irradiance profiles to determine the diffuse attenuation coefficient (K d , m -1 ), and the beam attenuation coefficient at 660 nm [c(660)]. The average exponential slope values for CDOM (0.0175nm -1 ) and NAP (0.0121nm -1 ) were nearly equivalent to, and spectral features of aΦ were similar to, those reported by Babin et al. (2003b) for coastal marine waters around Europe. Order of magnitude differences in study average values of a CDOM , a NAP , and a Φ were observed amongst the study lakes. The largest component in each of the lakes, for the commonly adopted reference wavelength of 440 nm, was CDOM; it represented from 48 to 68 % of a amongst the study lakes. The a NAP and a Φ components were approximately of equal importance, with a NAP (440) exceeding a Φ (440) in 6 of the lakes. The value of a, from summation of the measured components, was a strong predictor of K d differences amongst the lakes, and it had significant relationships with average values of SD and c(660). The ratio of the watershed area to the lake volume was found to be a strong predictor of the major differences in both a CDOM (440) and a NAP (440) amongst the lakes indicating the importance of terrigenous inputs of these constituent.

Phytoplankton absorption and the chlorophyll a–specific absorption coefficient in dynamic Onondaga Lake

Abstract Phytoplankton absorption and its dependence on the concentration of chlorophyll a(Chl-a), as represented by the Chl-a–specific absorption coefficient (a*φ(λ)), is important to support models of growth and for bio-optical remote sensing algorithms to retrieve Chl-a. The dynamics of the phytoplankton absorption coefficient (aφ(λ)) and a*φ(λ), and their dependencies on Chl-a, are described for Onondaga Lake, New York, over a 6-year period for which major changes in trophic state, Chl-a, and community composition occurred. Strong positive dependencies of aφ(λ) on Chl-aare reported for absorption peaks in both the blue and red spectral regions that are qualitatively similar to relationships for ocean waters but differ quantitatively. Average values of a*φ at wavelengths of 440 and 676 nm were 0.0347 and 0.0171 m2 mg−1, respectively, with coefficients of variation of 37 and 31%. Significant negative relationships between a*φ and Chl-a were observed for blue and green wavelengths that were qualitatively consistent with the influences of pigment packaging and the contribution of accessory pigments to absorption. The operation of these influences is demonstrated through various forms of data analysis that resolved the following significant relationships: (1) negative dependence of the ratio aφ(440):aφ(676) on Chl-a; (2) flattening of aφ spectra in the blue and increases at the red maximum, with increases in Chl-a; and (3) negative dependence of aφ(490):aφ(676) on Chl-a. Values of a*φ(440) and a*φ(676) obtained for Onondaga Lake are considered in the context of the limited population reported for other inland waters and selected marine systems.

Light Absorption Components in Onondaga Lake, New York, U.S.A.

Features of light absorption are critical to optical aspects of water quality and in regulating the signal available in the emergent flux from the surface. Spectral characteristics, dynamics, and relationships with optically active constituents and common optical metrics, are documented for three light absorbing components in culturally eutrophic Onondaga Lake, New York, U.S.A., based on the results of a 5 year study. The absorption coefficient, a(m -1 ), is partitioned according to the additive components of colored dissolved organic matter (a CDOM ), non-algal particles (a NAP ), phytoplankton (a φ ), and water itself (a w ; known), based on laboratory measurements for near surface samples collected weekly for the spring to fall interval over the 2004-2008 period. Supporting bulk measurements included dissolved organic carbon, total suspended solids (TSS) and its organic and inorganic (FSS) fractions, the concentrations of total phosphorus and chlorophyll-a ([Chl]), downwelling irradiance profiles to determine the diffuse attenuation coefficient (K d ), the beam attenuation coefficient at 660 nm [c(660)], and Secchi disc depth. The average exponential slope values for a CDOM (0.0168 nm -1 ) and a NAP (0.0126 nm -1 ) were very similar to those reported for other case 2 systems. The shapes of a φ spectra, represented by the ratio a φ (440): a φ (676), were found to negatively depend on [Chl]. The largest component of a at 440 nm, the reference wavelength, in each of the 5 years was a CDOM (∼45 to 60 %), the smallest was a NAP (∼15 to 25 %). Progressive decreases in yearly average a φ , particularly over the last three years are documented in response to decreases in [Chl], driven by increased nutrient limitation and Daphnia grazing (2008 only). Wide short-term variations are demonstrated within years for the components of a, particularly in response to the timing of runoff and Daphnia grazing events. Strong positive relationships are reported between a NAP and both TSS and FSS, a φ (at both λ = 440 and 676 nm) and [Chl], and a p (a NAP plus a φ ) and both TSS and c(660). The summation of the absorbing components at 440 nm is demonstrated to be a strong predictor of K d . The utility of the information in supporting a model for K d and SD and advancing monitoring of water quality through measurements of surface reflectance is described.

Linking CDOM patterns in Cayuga Lake, New York, USA, to terrigenous inputs

Abstract Lacustrine patterns of the light absorption of colored dissolved organic matter (aCDOM) and its composition proxies were resolved and linked to concurrent conditions of tributary inputs for Cayuga Lake, New York. We analyzed fixed-frequency samples of the lake at 3 sites and runoff event-based samples at the mouths of 3 gauged tributaries over a 7 month interval and measured dissolved organic carbon (DOC) and aCDOM over the visible wavelengths (400–700 nm) and at 254 nm. The tributaries are demonstrated to be enriched in aCDOM and DOC, with widely different proxy conditions compared to the lake, which further diverge during runoff events. DOC, aCDOM, and the composition proxies for the tributaries had significant, and mostly strong, dependencies on flow rate, described by power-law relationships. The differences in the composition proxies indicated lower contributions of CDOM to the DOC pool, reduced aromaticity, decreased molecular size of CDOM, and decreased amounts of humic versus fulvic acids in the lake compared to the tributaries, all accepted signatures of photobleaching. Dynamics of aCDOM in the upper waters of the lake depended primarily on composition (e.g., color quality) and secondarily on a quantity metric (DOC), as demonstrated in a 2-component linear least-squares regression format. Signatures of linkages between the terrestrial inputs and in-lake aCDOM patterns and the effects of photobleaching include (1) the preferential in-lake loss of aCDOM relative to DOC, estimated from budget calculations; (2) the intermediate characteristics resolved at a near-shore site adjoining multiple tributary inflows; and (3) the magnitude and charactero f the dynamics observed at the pelagic sites.

Optical characterization and tests of closure for Oneida Lake, New York, U.S.A.

Abstract Optical characteristics are a central feature of water quality and are increasingly used as a basis for monitoring concentrations of optically active constituents (OACs) through satellite-based remote sensing. This study presents a robust optical characterization of the near-surface waters of eutrophic Oneida Lake, New York, USA, that features in situ spectral measurements of inherent optical properties (IOPs) and remote sensing reflectance (Rrs(λ)), an apparent optical property (AOP) critical to remote sensing initiatives. The IOP measurements included the absorption coefficient, (a(λ)), and particulate scattering (bp(λ)) and backscattering (bbp(λ)) coefficients. Supporting measurements included Secchi depth (SD), the OACs, chlorophyll a, phycocyanin, suspended particulate material, and laboratory determinations of components of a(λ) (ax(λ)), including colored dissolved organic matter, nonalgal particles, and phytoplankton. The strong spectral patterns of a, ax, and Rrs, and more modest wavelength dependencies of bp and bbp, are documented. The credibility of the optical characterizations is demonstrated through (1) the consistency of relationships between SD−1 and bp, and ax and the OACs; (2) the extent of closure between in situ and laboratory measurements of a and ax; and (3) the extent of closure between predicted Rrs spectra, based on the observed IOP (a and bbp) spectra and application of a widely used radiative transfer relationship, and observations. Major temporal and substantial spatial variations in OACs, IOPs, SD, and Rrs(λ) are documented. Much of the variability in the OACs and SD were well predicted by system-specific empirical relationships that used Rrs values at specified wavelengths (consistent with the Medium Resolution Imaging Spectrophotometer [MERIS]) as the independent variables, supporting the potential for effective monitoring of these features through remote sensing in the future.

A Mechanistic Model for Secchi Disk Depth, Driven by Light Scattering Constituents

An optics theory-based mechanistic model for Secchi disk depth (ZSD) is advanced, tested, and applied for Cayuga Lake, NY. Robust data sets supported the initiative, including for (1) ZSD, (2) multiple light attenuation metrics, most importantly the beam attenuation (c) and particulate scattering (bp) coefficients, and (3) measures of constituents responsible for contributions to bp by phytoplankton (bo) and minerogenic particles (bm). The model features two serially connected links. The first link supports predictions of bp from those for bo and bm. The second link provides predictions of ZSD based on those for bp, utilizing an earlier optical theory radiative transfer equation. Recent advancements in mechanistically strong estimates of bm, empirical estimates of bo, and more widely available bulk measurements of c and bp have enabled a transformation from a theory-based conceptual to this implementable ZSD model for lacustrine waters. The successfully tested model was applied to quantify the contributions of phytoplankton biomass, and minerogenic particle groups, such as terrigenous clay minerals and autochthonously produced calcite, to recent bp and ZSD levels and dynamics. Moreover, it has utility for integration as a submodel into larger water quality models to upgrade their predictive capabilities for ZSD.

Light absorption by phytoplankton and minerogenic particles in Cayuga Lake, New York

Abstract Features of light absorption are critical determinants of the underwater and emergent light fields of lakes. Patterns in time and space of the components (ax) of the light absorption coefficient (a), and their dependence on various optically active constituents (OACs), in the form of material-specific ax, were resolved for Cayuga Lake, New York, USA. Component a was partitioned according to the additive components of ax that include phytoplankton (aϕ), nonalgal particles (aNAP), and colored dissolved organic material (aCDOM). Specific absorption coefficients were developed that couple the concentration of chlorophyll a (Chl-a, an OAC) to aϕ, and the concentration of inorganic suspended particulate material (ISPM, an OAC), or the projected area of minerogenic particles per unit volume of water (PAVm, an alternate OAC), to aNAP. The critical role of such specific coefficients in contemporary strategies to retrieve OACs through remote sensing for inland waters is described, and characteristics consistent with the influences of pigment packaging and accessory pigments on the aϕ–Chl-a relationship are demonstrated. Nonalgal partical aNAP is reported to be primarily regulated by the level of minerogenic particles. The specific absorption coefficients developed for Cayuga Lake to support retrieval of the OACs of Chl-a, PAVm, and ISPM are considered in the context of the limited populations of estimates available for inland waters.