Donald P. Morris

Chemical and optical changes in freshwater dissolved organic matter exposed to solar radiation

We studied the chemical and optical changes inthe dissolved organic matter (DOM) from twofreshwater lakes and a Sphagnum bog afterexposure to solar radiation. Stable carbonisotopes and solid-state 13C-NMR spectraof DOM were used together with optical andchemical data to interpret results fromexperimental exposures of DOM to sunlight andfrom seasonal observations of two lakes innortheastern Pennsylvania. Solar photochemicaloxidation of humic-rich bog DOM to smaller LMWcompounds and to DIC was inferred from lossesof UV absorbance, optical indices of molecularweight and changes in DOM chemistry. Experimentally, we observed a 1.2‰ enrichment in δ13

EFFECTS OF THE HERBICIDE ROUNDUP ON FRESHWATER MICROBIAL COMMUNITIES: A MESOCOSM STUDY

C and a 47% loss in aromaticC functionality in bog DOM samples exposed tosolar UVR. Similar results were observed inthe surface waters of both lakes. In latesummer hypolimnetic water in humic LakeLacawac, we observed 3 to 4.5‰enrichments in δ13C and a 30% increase inaromatic C relative to early spring valuesduring spring mixing. These changes coincidedwith increases in molecular weight and UVabsorbance. Anaerobic conditions of thehypolimnion in Lake Lacawac suggest thatmicrobial metabolism may be turning overallochthonous C introduced during springmixing, as well as autochthonous C. Thismetabolic activity produces HMW DOM during thesummer, which is photochemically labile andisotopically distinct from allochthonous DOM orautochthonous DOM. These results suggest bothphotooxidation of allochthonous DOM in theepilimnion and autotrophic production of DOM bybacteria in the hypolimnion cause seasonaltrends in the UV absorbance of lakes.

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

The impact of the widely used herbicide glyphosate has been mainly studied in terrestrial weed control, laboratory bioassays, and field studies focusing on invertebrates, amphibians, and fishes. Despite the importance of phytoplankton and periphyton communities at the base of the aquatic food webs, fewer studies have investigated the effects of glyphosate on freshwater microbial assemblages. We assessed the effect of the commercial formulation Roundup using artificial earthen mesocosms. The herbicide was added at three doses: a control (without Roundup) and two treatments of 6 and 12 mg/L of the active ingredient (glyphosate). Estimates of the dissipation rate (k) were similar in the two treatments (half-lives of 5.77 and 7.37 d, respectively). The only two physicochemical parameters showing statistically significant differences between treatments and controls were the downward vertical spectral attenuation coefficient kd(lambda), where lambda is wavelength, and total phosphorus concentration (TP). At the end of the experiment, the treated mesocosms showed a significant increase in the ratio kd(490 nm)/k(d)(550 nm) and an eightfold increase in TP. Roundup affected the structure of phytoplankton and periphyton assemblages. Total micro- and nano-phytoplankton decreased in abundance in treated mesocosms. In contrast, the abundance of picocyanobacteria increased by a factor of about 40. Primary production also increased in treated mesocosms (roughly by a factor of two). Similar patterns were observed in the periphytic assemblages, which showed an increased proportion of dead: live individuals and increased abundances of cyanobacteria (about 4.5-fold). Interestingly, the observed changes in the microbial assemblages were captured by the analysis of the pigment composition of the phytoplankton, the phytoplankton absorption spectra, and the analysis of the optical properties of the water. The observed changes in the structure of the microbial assemblages are more consistent with a direct toxicological effect of glyphosate rather than an indirect effect mediated by phosphorus enrichment.

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

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.

Optical Properties of Water

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

Light is critical in structuring aquatic ecosystems. Seasonal variations in the heat energy provided by sunlight are responsible for thermal stratification and mixing regimes of aquatic systems. Sunlight used in photosynthesis (both terrestrial and aquatic) forms the foundation of all but the simplest microbial communities of aquatic ecosystems. This article addresses the fate of light in aquatic ecosystems. It starts with a discussion of the nature of light and the spectral output of the sun. Reflectance, scattering, and absorption in the atmosphere are addressed, along with the role of latitude, in regulating the incident flux of light at the surface of aquatic systems. Once light penetrates the air–water interface, it can either be scattered or absorbed. The inherent optical properties of absorptance, scatterance, and volume scattering function are defined as they pertain to aquatic systems. Wavelength-specific differences in scattering and absorption are discussed in the context of optically significant components of the water column such as dissolved organic carbon and phytoplankton. The diffuse attenuation coefficient is introduced as a parameter in defining the transparency of aquatic systems. Wavelength-specific differences in this parameter are discussed and placed in the context of a systems inherent optical properties. Finally, the environmental variability of transparency is illustrated using data from five Western Hemisphere lakes.

Assessing changes in microbial respiration, bacterial growth efficiency, and bacterial production with nutrient addition to batch cultures

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.

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

Dissolved organic carbon (DOC) provides an important source of energy in lotic systems that can regulate food chain dynamics. It is thought that inorganic nutrient addition to stream culture samples will increase microbial respiration and bacterial growth efficiency (BGE). Nutrient limitation may prevent microbial communities from reaching their full potential in terms of biomass and efficiency, which could limit organisms at higher trophic levels. This study utilized batch cultures to assess changes across a variety of water quality and nutrient parameters in an effort to assess the role of inorganic nutrient addition in regulating microbial activity in headwater streams. Treatments of phosphorus addition (+ P) and nitrogen plus phosphorus addition (+ N + P) were used to assess changes in microbial activity in samples collected from forested and agricultural headwater streams. Phosphorus limitation was observed only at a single site, suggesting that a secondary limiting nutrient (e.g. nitrogen) may affect microbial activity at the other sites. Combined phosphorous and nitrogen additions demonstrated that nitrogen did limit microbial respiration at many of the study sites. BGE was highly variable and did not seem to be controlled by inorganic nutrient concentration. We did observe increased carbon production with nutrient addition at some of the study sites. The results of this study indicate that small streams could export more carbon through respiration rather than by incorporating available DOC into biomass.

The attenuation of solar UV radiation in lakes and the role of dissolved organic carbon

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