Bruce R. Hargreaves

BENEFICIAL AND DETRIMENTAL EFFECTS OF UV ON AQUATIC ORGANISMS: IMPLICATIONS OF SPECTRAL VARIATION

Solar ultraviolet radiation (UVR) may have beneficial as well as detrimental effects on living systems. For example, UV-B radiation (280-320 nm) is generally dam- aging, while UV-A radiation (320-400 nm) may cause damage or stimulate beneficial photorepair of UV-B damage. The nature of both direct and indirect effects of UVR in nature depends on both the photon flux density and the spectral composition of the radiation incident on aquatic organisms across environmental UVR gradients in space (depth, trans- parency, elevation) and time (diel, seasonal, interannual). Here we use the common and widespread freshwater cladoceran Daphnia pulicaria as a model organism to demonstrate the potential importance of these wavelength-specific effects of UVR to the ecology of aquatic organisms. UVR-exposure experiments are used to manipulate both natural solar and artificial UVR sources to examine the beneficial as well as detrimental effects of different wavelengths of UVR. Changes in the spectral composition of solar radiation are also examined along several natural environmental gradients including diel gradients, depth gradients, and dissolved organic carbon (DOC) gradients. The implications of variation in the spectral composition of UVR for aquatic organisms are discussed. The first biological weighting function (BWF) for a freshwater cladoceran is presented here. It demonstrates that the shortest UV-B wavelengths in sunlight are potentially the most damaging per photon. However, due to the greater photon flux density of longer wavelength UVR in sunlight, the net potential damage to Daphnia in nature is greatest for the longer wavelength UV-B and shorter wavelength UV-A radiation in the 305-322 nm range. Overall the contribution of UV-B to the total mortality response of Daphnia exposed to full-spectrum solar radiation for 7 h on a sunny summer day is 64% while UV-A con- tributes 36%. The BWF for Daphnia is used with the transmission spectrum for Mylar D to demonstrate that Mylar D cuts out only about half of the damaging UVR in sunlight. Following exposure to damaging UV-B, Daphnia exhibits a dramatic increase in survival in the presence of longer wavelength UV-A and visible radiation due to the stimulation of photoenzymatic repair. We present data that demonstrate the importance of both atmospheric ozone and DOC in creating strong environmental gradients in the intensity (irradiance) and spectral composition of solar UVR in nature. The light-absorbing component of DOC, chromophoric dissolved organic matter (CDOM), is particularly important in creating depth refugia from damaging UV-B in freshwater ecosystems. CDOM may also cause intense variations in the ratio of potentially beneficial UV-A to detrimental UV-B radiation to which aquatic organisms are exposed. In addition to changes in atmospheric ozone, future changes in CDOM related to climate change or other environmental disturbances may substantially alter the underwater exposure of a variety of aquatic organisms to different wavelengths of solar UVR.

Southern Ocean Gas Exchange Experiment: Setting the stage

[1] The Southern Ocean Gas Exchange Experiment (SO GasEx) is the third in a series of U.S.‐led open ocean process studies aimed at improving the quantification of gas transfer velocities and air‐sea CO2 fluxes. Two deliberate 3He/SF6 tracer releases into relatively stable water masses selected for large DpCO2 took place in the southwest Atlantic sector of the Southern Ocean in austral fall of 2008. The tracer patches were sampled in a Lagrangian manner, using observations from discrete CTD/Rosette casts, continuous surface ocean and atmospheric monitoring, and autonomous drifting instruments to study the evolution of chemical and biological properties over the course of the experiment. CO2 and DMS fluxes were directly measured in the marine air boundary layer with micrometeorological techniques, and physical, chemical, and biological processes controlling air‐sea fluxes were quantified with measurements in the upper ocean and marine air. Average wind speeds of 9 m s−1 to a maximum of 16 m s−1 were encountered during the tracer patch observations, providing additional data to constrain wind speed/gas exchange parameterizations. In this paper, we set the stage for the experiment by detailing the hydrographic observations during the site surveys and tracer patch occupations that form the underpinning of observations presented in the SO GasEx special section. Particular consideration is given to the mixed layer depth as this is a critical variable for estimates of fluxes and biogeochemical transformations based on mixed layer budgets.

Effects of urbanization on watershed hydrology: The scaling of discharge with drainage area

This study examines the effects of impervious surfaces within urbanized land on the scaling of river discharge with drainage area. Discharge in a river channel grows as drainage basin area increases following the general equation Q = kA c , where Q is river discharge, k is a measure of river base flow, A is upstream drainage area, and c is the scaling power dependency. Land use is a critical variable in the examination of river discharge; discharge has significant geologic and ecologic influences on fluvial systems. Discharge is assumed to scale linearly or nearly linearly with drainage area ( c ∼1), but in spite of its widespread application, the relationship has not been explicitly tested with respect to urbanization. Here we show that in small urban settings the scaling is nonlinear for peak flows. It is proposed that effective water loading occurs through a combination of increased runoff and an increase in the rate of transport to the rivers. These higher discharges in urban rivers have the potential to increase erosion, degrade aquatic habitats, and significantly alter channel forms.

Beneficial and detrimental interactive effects of dissolved organic matter and ultraviolet radiation on Zooplankton in a transparent lake

While changes in dissolved organic matter (DOM) concentrations are expected to affect zooplankton species through attenuation of potentially damaging ultraviolet (UV) radiation, generation of potentially beneficial or harmful photoproducts, pH alteration, and microbial food web stimulation, the combined effects of such changes on zooplankton community structure have not been studied previously. Our purpose was to determine how an increase in allochthonous DOM and associated changes in pH in an initially transparent lake may affect zooplankton community structure, and how exposure to solar UV may alter these DOM and pH effects. We ran microcosm experiments manipulating UV, DOM, and pH near the surface of Lake Giles in northeastern Pennsylvania. We found that when DOM was added in the presence of ambient UV, Daphnia and copepod UV-mortality was reduced by approximately three and two times compared to UV exposure without extra DOM. When DOM was added in the absence of UV, adult Daphnia and copepods were reduced compared to no DOM addition in the absence of UV. Daphnia and cyclopoid egg production and rotifer abundance were generally higher in the presence of DOM, regardless of UV treatment. The lower abundance yet high egg production in the presence of DOM and absence of UV may be explained by higher abundance of egg-bearing adults compared to non-egg-bearers. We conclude that allochthonous DOM benefits some zooplankton in a high-UV environment, but may be detrimental under low-UV conditions. Overall, Daphnia abundance and egg production were higher than that of calanoid copepods in the DOM additions, indicating that in some lakes an increase in allochthonous DOM may lead to a zooplankton community shift favoring Daphnia over calanoid copepods.

Estimating Attenuation of Ultraviolet Radiation in Streams: Field and Laboratory Methods

We adapted and tested a laboratory quantitative filter pad method and field‐based microcosm method for estimating diffuse attenuation coefficients (Kd) of ultraviolet radiation (UVR) for a wide range of stream optical environments (Kd320 = 3–44 m−1). Logistical difficulties of direct measurements of UVR attenuation have inhibited widespread monitoring of this important parameter in streams. Suspended sediment concentrations were manipulated in a microcosm, which was used to obtain direct measurements of diffuse attenuation. Dissolved and particulate absorption measurements of samples from the microcosm experiments were used to calibrate the laboratory method. Conditions sampled cover a range of suspended sediment (0–50 mg L−1) and dissolved organic carbon concentrations (1–4 mg L−1). We evaluated four models for precision and reproducibility in calculating particulate absorption and the optimal model was used in an empirical approach to estimate diffuse attenuation coefficients from total absorption coefficients. We field‐tested the laboratory method by comparing laboratory‐estimated and field‐measured diffuse attenuation coefficients for seven sites on the main stem and 10 tributaries of the Lehigh River, eastern Pennsylvania, USA. The laboratory‐based method described here affords widespread application, which will further our understanding of how stream optical environments vary spatially and temporally and consequently influence ecological processes in streams.

A simple toxicity apparatus for continuous flow with small volumes: demonstration with mysids and naphthalene

To determine the toxicity of naphthalene to the mysid crustacean Neomysis americana a small continuous-flow toxicant exposure system was built using design features of Mariotte bottle chemostats. Chemostats maintain cultures at constant cell concentrations by providing constant nutrient inputs. Such cultures remain well-mixed and have minimal cell contact with the container. The same characteristics are useful for exposing small invertebrates to volatile toxicants. The Mariotte bottles in the exposure system were modified by addition of air inlet restrictors. This modification provided stable flow at low pressure heads. The apparatus provided good water quality and physical environment for 10 mysids per chamber, and a more constant concentration of naphthalene than did static chambers replenished daily. Preliminary 96 h toxicity tests with N. americana were performed at 15/sup 0/C and 25/sup 0/C at concentrations of 900 and 1800 ..mu..g l/sup -1/ naphthalene in artificial seawater, for comparison with similar tests performed under static conditions. (JMT)

Effects of urbanization on watershed hydrology: The scaling of discharge with drainage area: COMMENT AND REPLY REPLY

We thank Criss and Winston (2007) for their interest in and analysis of our manuscript on the interaction between discharge and drainage area. We feel that their points help strengthen the conclusions of our original article ([Galster et al., 2006][1]). ‘k’ Values : Criss and Winston begin by