Charles L. Gallegos

Long‐term changes in light scattering in Chesapeake Bay inferred from Secchi depth, light attenuation, and remote sensing measurements

[1] The relationship between the Secchi depth (ZSD) and the diffuse attenuation coefficient for photosynthetically active radiation (Kd(PAR)), and in particular the product of the two, ZSD · Kd(PAR), is governed primarily by the ratio of light scattering to absorption. We analyzed measurements of ZSD and Kd(PAR) at main stem stations in Chesapeake Bay and found that the ZSD · Kd(PAR) product has declined at rates varying from 0.020 to 0.033 yr −1 over the 17 to 25 years of measurement, implying that there has been a long‐term increase in the scattering‐to‐absorption ratio. Remote sensing reflectance at the green wavelength most relevant to ZSD and Kd(PAR) in these waters, Rrs(555), did not exhibit an increasing trend over the 10 years of available measurements. To reconcile the observations we constructed a bio‐optical model to calculate ZSD, Kd(PAR), ZSD · Kd(PAR), and Rrs(555) as a function of light attenuating substances and their mass‐specific absorption and scattering coefficients. When simulations were based exclusively on changes in concentrations of light attenuating substances, a declining trend in ZSD · Kd entailed an increasing trend in Rrs(555), contrary to observations. To simulate both decreasing ZSD · Kd(PAR) and stationary Rrs(555), it was necessary to allow for a declining trend in the ratio of backscattering to total scattering. Within our simulations, this was accomplished by increasing the relative proportion of organic detritus with high mass‐specific scattering and low backscattering ratio. An alternative explanation not explicitly modeled is an increasing tendency for the particulate matter to occur in large aggregates. Data to discriminate between these alternatives are not available.

Long-Term Trends of Nutrients and Phytoplankton in Chesapeake Bay

Climate effects on hydrology impart high variability to water-quality properties, including nutrient loadings, concentrations, and phytoplankton biomass as chlorophyll-a (chl-a), in estuarine and coastal ecosystems. Resolving long-term trends of these properties requires that we distinguish climate effects from secular changes reflecting anthropogenic eutrophication. Here, we test the hypothesis that strong climatic contrasts leading to irregular dry and wet periods contribute significantly to interannual variability of mean annual values of water-quality properties using in situ data for Chesapeake Bay. Climate effects are quantified using annual freshwater discharge from the Susquehanna River together with a synoptic climatology for the Chesapeake Bay region based on predominant sea-level pressure patterns. Time series of water-quality properties are analyzed using historical (1945–1983) and recent (1984–2012) data for the bay adjusted for climate effects on hydrology. Contemporary monitoring by the Chesapeake Bay Program (CBP) provides data for a period since mid-1984 that is significantly impacted by anthropogenic eutrophication, while historical data back to 1945 serve as historical context for a period prior to severe impairments. The generalized additive model (GAM) and the generalized additive mixed model (GAMM) are developed for nutrient loadings and concentrations (total nitrogen—TN, nitrateu2009+u2009nitrate—NO2u2009+u2009NO3) at the Susquehanna River and water-quality properties in the bay proper, including dissolved nutrients (NO2u2009+u2009NO3, orthophosphate—PO4), chl-a, diffuse light attenuation coefficient (KD (PAR)), and chl-a/TN. Each statistical model consists of a sum of nonlinear functions to generate flow-adjusted time series and compute long-term trends accounting for climate effects on hydrology. We present results identifying successive periods of (1) eutrophication ca. 1945–1980 characterized by approximately doubled TN and NO2u2009+u2009NO3 loadings, leading to increased chl-a and associated ecosystem impairments, and (2) modest decreases of TN and NO2u2009+u2009NO3 loadings from 1981 to 2012, signaling a partial reversal of nutrient over-enrichment. Comparison of our findings with long-term trends of water-quality properties for a variety of estuarine and coastal ecosystems around the world reveals that trends for Chesapeake Bay are weaker than for other systems subject to strenuous management efforts, suggesting that more aggressive actions than those undertaken to date will be required to counter anthropogenic eutrophication of this valuable resource.

Scientific Bases for Numerical Chlorophyll Criteria in Chesapeake Bay

In coastal ecosystems with long flushing times (weeks to months) relative to phytoplankton growth rates (hours to days), chlorophyll a (chl-a) integrates nutrient loading, making it a pivotal indicator with broad implications for ecosystem function and water-quality management. However, numerical chl-a criteria that capture the linkage between chl-a and ecosystem impairments associated with eutrophication (e.g., hypoxia, water clarity and loss of submerged aquatic vegetation, toxic algal blooms) have seldom been developed despite the vulnerability of these ecosystems to anthropogenic nutrient loading. Increases in fertilizer use, animal wastes, and population growth in the Chesapeake Bay watershed since World War II have led to increases in nutrient loading and chl-a. We describe the development of numerical chl-a criteria based on long-term research and monitoring of the bay. Baseline chl-a concentrations were derived using statistical models for historical data from the 1960s and 1970s, including terms to account for the effects of climate variability. This approach produced numerical chl-a criteria presented as geometric means and 90th percentile thresholds to be used as goals and compliance limits, respectively. We present scientific bases for these criteria that consider specific ecosystem impairments linked to increased chl-a, including low dissolved oxygen (DO), reduced water clarity, and toxic algal blooms. These multiple lines of evidence support numerical chl-a criteria consisting of seasonal mean chl-a across salinity zones ranging from 1.4 to 15xa0mgxa0m−3 as restoration goals and corresponding thresholds ranging from 4.3 to 45xa0mgxa0m−3 as compliance limits. Attainment of these goals and limits for chl-a is a precondition for attaining desired levels of DO, water clarity, and toxic phytoplankton prior to rapid human expansion in the watershed and associated increases of nutrient loading.

Dependence of eelgrass (Zostera marina) light requirements on sediment organic matter in Massachusetts coastal bays: implications for remediation and restoration.

Using a calibrated bio-optical model we determined that the optical water quality conditions in several nitrogen-impaired embayments and in one unimpaired system were within the range of values known to support eelgrass growth. We also used the model to identify a range of light requirements for eelgrass (Zostera marina). Higher eelgrass light requirements, expressed as a percentage of surface-incident irradiance, corresponded with higher sediment organic matter content. These results corroborated findings by previous studies which indicate a generalized relationship: seagrasses growing in turbid conditions with poorer water and sediment quality have higher light requirements than those growing in less degraded conditions. The mechanistic reason for the variation in light requirements is still not completely explained and cannot be attributed to a single independent variable. Varying light requirement have important implications for eelgrass protection and should be considered when setting restoration targets for eelgrass in water quality and nitrogen remediation programs.

Optical Water Quality of Inland Waters: A Landscape Perspective

Optical water quality (OWQ) relates the composition of waters to their light transmission and thereby light availability for aquatic plants, visual range for aquatic animals, and suitability for recreational uses. This fundamental role of OWQ has led to numerous studies on the optical properties in rivers and lakes, which we overview here with a landscape perspective. International examples illustrate how the kinds and amounts of light-attenuating substances depend on landscape features on scales ranging from the river reach to the orientation of mountain ranges. OWQ is profoundly affected by increased nutrient discharges and sediment mobilization from human activities such as agriculture. Proper monitoring of OWQ is needed to fully understand and address these consequences. Remote sensing has potential for broad-scale surveys of OWQ, although currently its applications to inland waters are limited by low spatial resolution. A better understanding of OWQ will advance water management and remote sensing capabilities, as well as provide further insight into water quality impacts associated with global changes in climate, energy use, and land use.

Variable climatic conditions dominate recent phytoplankton dynamics in Chesapeake Bay

Variable climatic conditions strongly influence phytoplankton dynamics in estuaries globally. Our study area is Chesapeake Bay, a highly productive ecosystem providing natural resources, transportation, and recreation for nearly 16 million people inhabiting a 165,000-km2 watershed. Since World War II, nutrient over-enrichment has led to multiple ecosystem impairments caused by increased phytoplankton biomass as chlorophyll-a (chl-a). Doubled nitrogen (N) loadings from 1945–1980 led to increased chl-a, reduced water clarity, and low dissolved oxygen (DO), while decreased N loadings from 1981–2012 suggest modest improvement. The recent 30+ years are characterized by high inter-annual variability of chl-a, coinciding with irregular dry and wet periods, complicating the detection of long-term trends. Here, we synthesize time-series data for historical and recent N loadings (TN, NO2u2009+u2009NO3), chl-a, floral composition, and net primary productivity (NPP) to distinguish secular changes caused by nutrient over-enrichment from spatio-temporal variability imposed by climatic conditions. Wet years showed higher chl-a, higher diatom abundance, and increased NPP, while dry years showed lower chl-a, lower diatom abundance, and decreased NPP. Our findings support a conceptual model wherein variable climatic conditions dominate recent phytoplankton dynamics against a backdrop of nutrient over-enrichment, emphasizing the need to separate these effects to gauge progress toward improving water quality in estuaries.