A simple empirical optical model for simulating light attenuation variability in a partially mixed estuary

Representation of the subsurface light field is a crucial component of pelagic ecosystem and water quality models. Modeling the light field in estuaries is a particularly complicated problem due to the significant influence of high concentrations of dissolved and particulate matter that are derived from both terrestrial and estuarine sources. The goal of this study was to develop a relatively simple but effective way to model light attenuation variability in a turbuid estuary (Chesapeake Bay, United States) in a coupled physical-biological model. We adopted a simple, nonspectral empirical approach. Surface water quality data (salinity was used as a proxy of chromophoric dissolved organic matter [CDOM]) and light measurements from the Chesapeake Bay Program were used to determine the absorption coefficients in a linear attenuation model using regression methods. This model predicts K_c (specific attenuation due to phytoplankton/chlorophylla [chla]), K_t (specific attenuation due to total suspended solids), and K_s (a function of specific attenuation coefficients of CDOM in relation to salinity). The Bay-wide fitted relation between the light attenuation coefficient and water quality concentrations gives generally good estimates of total light attenuation, K_d. The direct inclusion of salinity in the relationship has one disadvantage: it can yield negative values for K_d at high salinities. We developed two separate models for two different salinity regimes. This approach, in addition to solving the negative K_d problem, also accounts for some changes in specific light absorption by chla, seston (nonphytoplankton particulate matter), and CDOM that apparently occur in different salinity regimes in Chesapeake Bay. The resulting model predicts the statistical characteristics (i.e., the mean and variance) of K_d quite accurately in most regions of Chesapeake Bay. We also discuss in this paper the feasibility and caveats of using K_d converted from Secchi depth in the empirical method.     

Impact of the Spring 2000 phytoplankton bloom in Chesapeake Bay on optical properties and light penetration in the Rhode River,Maryland

Accelerating eutrophication manifest as increasing frequency and magnitude of phytoplankton blooms threatens living resources in many estuaries. Effects of large blooms can be difficult to document because blooms are often unexpected and do not always coincide with scheduled sampling programs. Here we use continuously monitored salinity distributions and optical properties to study the spring bloom of the red tide dinoflagellate,Prorocentrumminimum, in the Rhode River, Maryland, a tributary embayment of upper Chesapeake Bay. Salinity distributions, together with weekly cruise measurements of nutrient concentrations, indicate that the bloom commenced with an influx of nitrate at the mouth due to the arrival of a freshet from the Susquehanna River. Arrival of this freshet at the mouth set up an unstable, inverse salinity gradient within the Rhode River. Continuously monitored absorption and scattering spectra indicated that increases in chlorophyll within the Rhode River initially were due to the influx of chlorophyll that had developed in the main stem of the bay. After the influx, much higher concentrations and steep spatial gradients developed within the Rhode River, subsequent to reduced mixing that accompanied re-establishment of a normal estuarine salinity gradient. We used the monitored absorption and scattering coefficients to determine the effect of the bloom on light attenuation coefficients in the Rhode River. The bloom resulted in a nearly three-fold increase in attenuation coefficient. Attenuation was dominated by chlorophyll in the early stages of the bloom and by detritus after the termination of the bloom. Although the bloom lasted only 20 d, the elevated attenuation coefficients due to the bloom exceeded values that would permit growth of submersed vascular plants for a period of about 45 d.     

Role of Late Winter–Spring Wind Influencing Summer Hypoxia in Chesapeake Bay

We examined the processes influencing summer hypoxia in the mainstem portion of Chesapeake Bay. The analysis was based on the Chesapeake Bay Monitoring Program data collected between 1985 and 2007. Self-organizing map (SOM) analysis indicates that bottom water dissolved oxygen (DO) starts to be depleted in the upper mesohaline area during late spring, and hypoxia expands down-estuary by early summer. The seasonal hypoxia in the bay appears to be related to multiple variables, (e.g., river discharge, nutrient loading, stratification, phytoplankton biomass, and wind condition), but most of them are intercorrelated. The winter–spring Susquehanna River flow contributes to not only spring–summer buoyancy effects on estuarine circulation dynamics but also nutrient loading from the land-promoting phytoplankton growth. In addition, we found that summer hypoxia is significantly correlated with the late winter–spring (February–April) northeasterly–southwesterly (NE–SW) wind. Based on winter–spring (January–May) conditions, a predictive tool was developed to forecast summer (June–August) hypoxia using river discharge and NE–SW wind. We hypothesized that the late winter–spring wind pattern may affect the transport of spring bloom biomass to the western shoal or the deep channel of the bay that either alleviates or increases the summer hypoxic volume in the midbay region, respectively. To examine this hypothesis, residual flow fields were analyzed using a hydrodynamic ocean model (Regional Ocean Modeling System; ROMS) between 2000 and 2003, two hydrologically similar years but years with different wind conditions during the spring bloom period. Simulation model results suggest that relatively larger amounts of organic matter could be transported into the deep channel in 2003 (severe hypoxia; frequent northeasterly wind) than 2000 (moderate hypoxia; frequent southwesterly wind).     

Sources of Fatty Acids in Sediments of the York River Estuary: Relationships with Physical and Biological Processes

Fatty acid (FA) composition (neutral FA and phospholipid-linked FA) was examined in surface sediments at two sites of the York River estuary (VA, Chesapeake Bay) with contrasting benthic dynamics, Clay Bank (CB) and Gloucester Point (GP), in order to study organic carbon sources and their relationship to biological and physical processes. Our results suggest that FA composition in surface sediments was driven by biological and physical events, occurring mostly during the period of high river discharge, including: (1) the spring phytoplankton bloom, likely responsible for high concentrations of FA of algal origin at CB in March 2007, (2) the fall phytoplankton bloom, causing a secondary peak of algal FA and an increase in viable microbial biomass at CB in October 2007, and (3) a physical event in May 2007, resulting in high concentrations of terrestrial and bacterial FA at GP. FA concentrations tended to be higher but more variable during periods of high freshwater discharge, with trends more pronounced at the upstream site, suggesting connectivity between biological and physical processes.     

Coupling Between the Coastal Ocean and Yaquina Bay,Oregon: Importance of Oceanic Inputs Relative to Other Nitrogen Sources

Understanding of the role of oceanic input in nutrient loadings is important for understanding nutrient and phytoplankton dynamics in estuaries adjacent to coastal upwelling regions as well as determining the natural background conditions. We examined the nitrogen sources to Yaquina Estuary (Oregon, USA) as well as the relationships between physical forcing and gross oceanic input of nutrients and phytoplankton. The ocean is the dominant source of dissolved inorganic nitrogen (DIN) and phosphate to the lower portion of Yaquina Bay during the dry season (May through October). During this time interval, high levels of dissolved inorganic nitrogen (primarily in the form of nitrate) and phosphate entering the estuary lag upwelling favorable winds by 2 days. The nitrate and phosphate levels entering the bay associated with coastal upwelling are correlated with the wind stress integrated over times scales of 4–6 days. In addition, there is a significant import of chlorophyll a to the bay from the coastal ocean region, particularly during July and August. Variations in flood-tide chlorophyll a lag upwelling favorable winds by 6 days, suggesting that it takes this amount of time for phytoplankton to utilize the recently upwelled nitrogen and be transported across the shelf into the estuary. Variations in water properties determined by ocean conditions propagate approximately 11–13 km into the estuary. Comparison of nitrogen sources to Yaquina Bay shows that the ocean is the dominant source during the dry season (May to October) and the river is the dominant source during the wet season with watershed nitrogen inputs primarily associated with nitrogen fixation on forest lands.     

Organic Matter Sources Supporting Lower Food Web Production in the Tidal Freshwater Portion of the York River Estuary, Virginia

The Mattaponi River is part of the York River estuary in Chesapeake Bay. Our objective was to identify the organic matter (OM) sources fueling the lower food web in the tidal freshwater and oligohaline portions of the Mattaponi using the stable isotopes of carbon (C) and nitrogen (N). Over 3 years (2002–2004), we measured zooplankton densities and C and N stable isotope ratios during the spring zooplankton bloom. The river was characterized by a May–June zooplankton bloom numerically dominated by the calanoid copepod Eurytemora affinis and cladocera Bosmina freyi. Cluster analysis of the stable isotope data identified four distinct signatures within the lower food web: freshwater riverine, brackish water, benthic, and terrestrial. The stable isotope signatures of pelagic zooplankton, including E. affinis and B. freyi, were consistent with reliance on a mix of autochthonous and allochthonous OM, including OM derived from vascular plants and humic-rich sediments, whereas macroinvertebrates consistently utilized allochthonous OM. Based on a dual-isotope mixing model, reliance on autochthonous OM by pelagic zooplankton ranged from 20% to 95% of production, declining exponentially with increasing river discharge. The results imply that discharge plays an important role in regulating the energy sources utilized by pelagic zooplankton in the upper estuary. We hypothesize that this is so because during high discharge, particulate organic C loading to the upper estuary increased and phytoplankton biomass decreased, thereby decreasing phytoplankton availability to the food web.     

Phytoplankton reference communities for Chesapeake Bay and its tidal tributaries

Phytoplankton reference communities for Chesapeake Bay were quantified from least-impaired water quality conditions using commonly measured parameters and indicators derived from measured parameters. A binning approach was developed to classify water quality. Least-impaired conditions had relatively high water column transparency and low concentrations of dissolved inorganic nitrogen and orthophosphate. Reference communities in all seasons and salinity zones are characterized by consistently low values of chlorophylla and pheophytin coupled with relative stable proportions of the phytoplankton taxonomic groups and low biomasses of key bloom-forming species. Chlorophyll cell content was lower and less variable and average cell size and seasonal picophytoplankton biomass tended to be greater in the mesohaline and polyhaline reference communities as compared to the impaired communities. Biomass concentrations of the nano-micro phytoplankton size fractions (2–200 μm) in 12 of the 16 season-specific and salinity-specific reference communities were the same or higher than those in impaired habitat conditions, suggesting that nutrient reductions will not decrease the quantity of edible phytoplankton food available to large consumers. High (bloom) and low (bust) biomass events within the impaired phytoplankton communities showed strikingly different chlorophyll cell content and turnover rates. Freshwater flow had little effect on phytoplankton responses to water quality condition in most of the estuary. Improved water column transparency, or clarity, through the reduction of suspended sediments will be particularly important in attaining the reference communities. Significant nitrogen load reductions are also required.     

Dissolved and particulate organic carbon in Chesapeake Bay

We measured dissolved and particulate organic carbon (DOC and POC) in samples collected along 13 transects of the salinity gradient of Chesapeake Bay. Riverine DOC and POC end-members averaged 232±19 μM and 151±53 μM, respectively, and coastal DOC and POC end-members averaged 172±19 μM and 43±6 μM, respectively. Within the chlorophyll maximum, POC accumulated to concentrations 50–150 μM above those expected from conservative mixing and it was significantly correlated with chlorophylla, indicating phytoplankton origin. POC accumulated primarily in bottom waters in spring, and primarily in surface waters in summer. Net DOC accumulation (60–120 μM) was observed within and downstream of the chlorophyll maximum, primarily during spring and summer in both surface and bottom waters, and it also appeared to be derived from phytoplankton. In the turbidity maximum, there were also net decreases in chlorophylla (?3 μg l^?1 to ?22 μg l^?1) and POC concentrations (?2 μM to ?89 μM) and transient DOC increases (9–88 μM), primarily in summer. These occurred as freshwater plankton blooms mixed with turbid, low salinity seawater, and we attribute the observed POC and DOC changes to lysis and sedimentation of freshwater plankton. DOC accumulation in both regions of Chesapeake Bay was estimated to be greater than atmospheric or terrestrial organic carbon inputs and was equivalent to ≈10% of estuarine primary production.     

Organic matter quality and supply to deep-water coral/mound systems of the NW European Continental Margin

Comparison of five deep-water coral (DWC)/mound ecosystems along the European Continental Margin shows that suspended particulate organic matter (sPOM), a potential food source, is lipid rich and of high quality. However, there are differences between the sites. The Darwin and Pelagia Mounds (N. Rockall Trough and N. Porcupine Bank, respectively) have higher proportions of labile particulate lipids (including high proportions of polyunsaturated fatty acids) in the benthic boundary layer than Logachev, Hovland and Belgica Mounds (Rockall Bank, S. Porcupine Bank and Porcupine Seabight, respectively). The high quality sPOM could be transported downslope from the euphotic zone. There is some evidence for inter-annual variability at some sites (e.g. Hovland and Logachev Mounds) as large differences in suspended lipid and particulate organic carbon concentrations were observed over the sampling period. Elevated total organic carbon contents of sediments at mound sites, relative to control sites in some cases (particularly Darwin Mounds), probably reflect local hydrodynamic control and the trapping of sPOM by the DWC. Fresh POM can be relatively rapidly transferred to significant depth (up to 8 cm) through bioturbation that is evident at all sites. There is no clear evidence of present day hydrocarbon seepage at any of the sites.     

Iron Budget and Its Correlations with Macronutrients in the Inshore Waters of the Aegean Sea

Macronutrients and micronutrients were measured during the phytoplankton bloom period and then seasonally monitored after the bloom in the polluted Izmir Bay. Iron and the macronutrients (phosphate, ammonium, nitrate, nitrite, and silicate) were abundant in the waters of the inner and middle sections of Izmir Bay. The iron concentration decreased exponentially from the eutrophic inner bay to the oligotrophic outer bay. Suboxic–anoxic processes and the resuspension dynamics in the sediment were the most important factor in the control of iron, ammonium, and phosphate enrichment in the bay beside the anthropogenic activities. The biological removal of Fe in the inner and middle bay and nonbiological removal in the outer bay were effective in controlling iron concentration in Izmir Bay. The nitrate, nitrite, and ammonium nitrogen (N) and Si decreased to critical levels in the middle and outer bay at the end of the summer as long as the concentration of phosphate was high. The N/P ratios in the bay suggested that N might be the controlling nutrient for phytoplankton growth particularly in the middle and outer bay throughout summer. Furthermore, Si was also able to have controlling impact probably on diatom growth during autumn and winter in the inner and middle bay and in the early spring in the outer bay. The N/Si/Chelex labile Fe ratios implied that the iron could be a critical controlling nutrient for phytoplankton growth during early April in the outer bay unless the other macronutrients were low.     

Phytoplankton biomass in a subtropical estuary: Distribution,size composition,and carbon:Chlorophyll ratios

The seasonal pattern of phytoplankton biomass (chlorophyll and particulate organic carbon) and the salinity-related pattern of phytoplankton biomass and size composition were determined in Apalachicola Bay, Florida, throughout 2004. Phytoplankton biomass was highest during summer and lowest during winter. During summer, phytoplankton biomass was highest in waters with salinity between about 5 and 23. In waters between 5 and 23, phytoplankton biomass was primarily (> 50%) composed of < 5 μm cells. The results from this study support the idea that a microbial food web characterizes mass and energy flow through the planktonic food web in Apalachicola Bay and other estuaries. During winter, the carbonxhlorophylla ratio averaged 56 ± 60 (standard deviation). During summer, the ratio ranged from 23 to 345, with highest values occurring in waters with salinity between about 8 and 22. The carbonxhlorophylla ratio was positively related to the percent of chlorophyll < 5 μm in size during summer.     

Scales of nutrient-limited phytoplankton productivity in Chesapeake Bay

The scales on which phytoplankton biomass vary in response to variable nutrient inputs depend on the nutrient status of the plankton community and on the capacity of consumers to respond to increases in phytoplankton productivity. Overenrichment and associated declines in water quality occur when phytoplankton growth rate becomes nutrient-saturated, the production and consumption of phytoplankton biomass become uncoupled in time and space, and phytoplankton biomass becomes high and varies on scales longer than phytoplankton generation times. In Chesapeake Bay, phytoplankton growth rates appear to be limited by dissolved inorganic phosphorus (DIP) during spring when biomass reaches its annual maximum and by dissolved inorganic nitrogen (DIN) during summer when phytoplankton growth rates are highest. However, despite high inputs of DIN and dissolved silicate (DSi) relative to DIP (molar ratios of N∶P and Si∶P>100), seasonal accumulations of phytoplankton biomass within the salt-intruded-reach of the bay appear to be limited by riverine DIN supply while the magnitude of the spring diatom bloom is governed by DSi supply. Seasonal imbalances between biomass production and consumption lead to massive accumulations of phytoplankton biomass (often>1,000 mg Chl-a m^?2) during spring, to spring-summer oxygen depletion (summer bottom water <20% saturation), and to exceptionally high levels of annual phytoplankton production (>400 g m^?2 yr^?1). Nitrogen-dependent seasonal accumulations of phytoplankton biomass and annual production occur as a consequence of differences in the rates and pathways of nitrogen and phosphorus cycling within the bay and underscore the importance of controlling nitrogen inputs to the mesohaline and lower reaches of the bay.     

Habitat Affects Survival of Translocated Bay Scallops, Argopecten irradians concentricus (Say 1822), in Lower Chesapeake Bay

Bay scallop (Argopecten irradians) populations existed in Chesapeake Bay until 1933, when they declined dramatically due to a loss of seagrass habitat. Since then, there have been no documented populations within the Bay. However, some anecdotal observations of live bay scallops within the lower Bay suggest that restoration of the bay scallop is feasible. We therefore tested whether translocated adults of the southern bay scallop, Argopecten irradians concentricus, could survive during the reproductive season in vegetated and unvegetated habitats of the Lynnhaven River sub-estuary of lower Chesapeake Bay in the absence of predation. Manipulative field experiments evaluated survival of translocated, caged adult scallops in eelgrass Zostera marina, macroalgae Gracilaria spp., oyster shell, and rubble plots at three locations. After a 3-week experimental period, scallop survival was high in vegetated habitats, ranging from 98% in their preferred habitat, Z. marina, to 90% in Gracilaria spp. Survival in Z. marina was significantly higher than that in rubble (76%) and oyster shell (78%). These findings indicate that reproductive individuals can survive in vegetated habitats of lower Chesapeake Bay when protected from predators and that establishment of bay scallop populations within Chesapeake Bay may be viable.     

梅梁湾、大太湖夏季和冬季CDOM特征及可能来源分析

基于2004年夏季水华暴发期和冬季在梅梁湾及大太湖各2次采样,分析了夏季、冬季CDOM的特征及其可能的来源,发现夏季CDOM吸收系数、叶绿素a浓度均明显高于冬季,DOC浓度、CDOM吸收系数a(355)的变化范围分别为5.17～12.42 mg/L、2.57～6.77 m-1,最大值均出现在冬季(12月15日)的直湖港入湖口.CDOM吸收系数与DOC浓度、定标后的荧光值一般都存在显著正相关,但夏季由于受浮游植物降解的影响,与DOC浓度和荧光的相关性明显低于冬季.表征CDOM组成和来源的参数比吸收系数、M值、S值存在显著的季节差异,夏季吸收系数a*(355)值明显要大于冬季,而S值、M值则要小于冬季.夏季水华暴发时CDOM吸收系数与叶绿素a浓度空间分布较为一致,吸收系数与叶绿素a浓度存在正相关,浮游植物降解产物可能是水体中CDOM的重要来源;相反,冬季CDOM吸收系数呈现从梁溪河入湖口、湾内往湾口递减的趋势,其来源可能主要以陆源为主,受入湖河流的影响较大.     

Transformation of phosphorus in the Elbe estuary

The distribution of dissolved and particle-bound phosphorus (P) was investigated in the Elbe estuary during March 1995. The forms of particulate P were studied with a sequential extraction technique. Organic P dominated particle-bound P in the outer reaches of the estuary (52%), decreased to a minimum of 21% in the turbidity zone, and increased to 33% further upstream. Fe-bound P was the second most important P species in the outer reaches (27%) and dominated in the turbidity zone (up to 57%) and upstream of the turbidity zone (up to 48%). The P:Fe ratio increased with decreasing salinity, from 0.11 in the outer reaches to about 0.22 at zero salinity. Dissolved inorganic P release from reverine suspended matter was about two to three times larger than release, from marine suspended matter and was dominated by release of Fe-bound P. Dissolved inorganic P release from marine and from riverine organic matter were of equal importance. Because marine suspended matter dominates in the estuary, this suggests riverine organic matter is remineralized much faster than marine organic matter. This is in line with the refractory nature of marine organic matter (no phytoplankton bloom) and the easily degradable character of the riverine suspended matter (phytoplankton bloom) in the Elbe estuary during March 1995.     

Impacts of SW Monsoon on Phytoplankton Community Structure Along the Western Coastal BOB: an HPLC Approach

The major Indian rivers bring significant amount of freshwater along with inorganic nutrients and sediment load in to the northern Bay of Bengal (BOB) during the southwest monsoon (SWM); the southern bay does not experience equal freshening. This contrasting pattern may considerably impact the physicochemical features and phytoplankton community composition in this bay and was investigated during a coastal cruise during the SWM covering eight river plumes from both northern and southern bay; phytoplankton pigments and physicochemical parameters were analysed from different depths (0, 10, 25, and 50 m). Significant freshening, stratification and warmer waters were noticed in the northern bay relative to its southern part. Phytoplankton pigment analysis and diagnostic pigment-based size class analysis revealed the dominance of microphytoplankton (mainly diatoms) in the northern bay and were mostly confined to the surface waters. Their abundance was positively correlated with dissolved silicate (DSi) concentrations and inversely with salinity. Nanophytoplankton and picophytoplankton (prymnesiophytes, chrysophytes and cyanophytes) were mostly noticed in the subsurface waters and dominated the southern bay. This finding suggests that the dominance of microphytoplankton in the northern bay may significantly contribute to higher particle flux which has been reported earlier. Therefore, any modification in future river discharge, which is in turn related to the intensity of Indian summer monsoon, will alter the phytoplankton community structure in the coastal BOB and may be further cascaded to the other vital ecosystem components like fisheries resources, organic carbon export flux and benthic production.     

Nutrient limitation of heterotrophic bacteria in Florida Bay

We examined heterotrophic bacterial nutrient limitation at four sites in Florida Bay, U. S. in summer 1994 and winter 1995. Bacterial growth and biomass production in this system were most limited by inorganic phosphorus (P) in the eastern and southern regions of the bay. Nutrient additions stimulated productivity and biomass accumulation mostly in summer. The magnitude of growth responses (thymidine incorporation) to nutrient additions was nearly an order of magnitude less in winter than summer. Biomass-normalized alkaline phosphatase activity in the northeast and south-central region was 5–20 times greater than in the northwest and north-central regions, suggesting that P is most limiting to planktonic growth in those areas. Chlorophyll levels were higher in the northwest and north-central regions and P-uptake into particles >1 μm, primarily phytoplankton, was also higher in these regions. Consistent with these observations, others have observed that P is advected into the bay primarily in the northwestern region. Abundant seagrasses in Florida Bay may promote heterotrophic bacterial production relative to phytoplankton production by releasing dissolved organic carbon that makes bacteria more competitive for limiting quantities of inorganic phosphate, especially in the eastern bay where turbidity is low, P is most limiting, and light levels reaching the benthic plants are high.     

Distribution and significance of chlorophyll derivatives and oxidation products during the spring phytoplankton bloom in the Celtic Sea April 2002

The abundances and distributions of chlorophyll a and its common transformation products have been profiled by high performance liquid chromatography during the disruption, re-establishment, growth and senescence of a phytoplankton bloom in the Celtic Sea during April 2002. Transformation products of chlorophyll a indicative of herbivory of the phytoplankton bloom have been identified using atmospheric pressure chemical ionisation liquid chromatography-multistage mass spectrometry and chlorophyll oxidation products, formed by autoxidation, are present in increasing abundance in samples collected during the later stages of bloom development. The formation of oxidation products during the earliest stages in the chlorophyll transformation pathway identifies water column processes as sources of these early intermediates in the formation of geologic aetioporphyrins and/or cycloalkanoporphyrins.     

The trophic consequences of oyster stock rehabilitation in Chesapeake Bay

There is mounting speculation that overharvesting of oyster stocks (Crassostrea virginica) in Chesapeake Bay may be a factor contributing to the decline in water quality and shifts in the dominance of species inhabiting the estuary. The trophic consequences of increasing the oyster population may be addressed using a simple quasi-equilibrium, mass action model of the exchanges transpiring in the Chesapeake mesohaline ecosystem. According to output from the model, increasing oyster abundance would decrease phytoplankton productivity as well as stocks of pelagic microbes, ctenophores, medusae, and particulate organic carbon. Recently acquired field data on phytoplankton productivity, bacterioplankton, and labile organic carbon in the vicinity of rafted oyster aquaculture support model predictions. The model also indicates that more oysters should increase benthic primary production, fish stocks, and mesozooplankton densities. Hence, augmenting the oyster community by restoring beds or introducing raft culture represents a potentially significant adjunct to the goal of mitigating eutrophication through curtailment of nutrient inputs. *** DIRECT SUPPORT *** A01BY059 00005     

Refining habitat requirements of submersed aquatic vegetation: Role of optical models

A model of the spectral diffuse attenuation coefficient of downwelling irradiance was constructed for Chincoteague Bay, Maryland, and the Rhode River, Maryland. The model is written in terms of absorption spectra of dissolved yellow substance, the chlorophyll-specific absorption of phytoplankton, and absorption and scattering by particulate matter (expressed as turbidity). Based on published light requirements for submersed aquatic vegetation (SAV) in Chesapeake Bay, the model is used to calculate the range of water-quality conditions that permit survival of SAV at various depths. Because the model is spectrally based, it can be used to calculate the attenuation of either photosynthetically active radiation (PAR, equally weighted quanta from 400 nm to 700 nm) or photosyntheticallyusable radiation (PUR, the integral of the quantum spectrum weighted by the pigment absorption spectrum of SAV). PUR is a more accurate measurement of light that can be absorbed by SAV and it is more strongly affected by phytoplankton chlorophyll in the water column than is PAR. For estuaries in which light attenuation is dominated by turbidity and chlorophyll, the model delimits regions in which turbidity alone (chlorophyll <10 μg 1^?1), chlorophyll alone (turbidity <1 NTU) or both factors (chlorophyll >10 μg 1^?1, turbidity >1 NTU) must be reduced to improve survival depths for SAV.