Robert E. Moeller

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

SOLAR ULTRAVIOLET RADIATION AND THE SPAWNING HABITAT OF YELLOW PERCH, PERCA FLAVESCENS

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.

Temperature Effects on Survival and DNA Repair in Four Freshwater Cladoceran Daphnia Species Exposed to UV Radiation

Dissolved organic carbon (DOC) strongly influences the underwater levels of potentially damaging solar ultraviolet radiation (UVR) in freshwater lakes. Even so, little is known about how DOC-related variation in UVR may influence natural populations and communities in lakes. Past studies of fish recruitment have emphasized the importance of temperature, food limitation, and predation in controlling year-class strength in fish. Here we report that high UVR levels in low-DOC lakes also may modify the spawning depth, hatching success, and thus recruitment of certain freshwater fishes. We examined how UVR influences the spawning habitat and hatching success of yellow perch (Perca flavescens) eggs in two lakes with different DOC levels and thus different UVR environ- ments. Yellow perch eggs were incubated at the same depth (0.8 m) in quartz tubes (with mesh ends for water exchange) in both lakes in a modified reciprocal transplant experiment. Solar radiation was manipulated to provide three treatments that included exposure to full solar radiation (quartz alone), shielding from UV-B with wavelength selective filters (Mylar D), and dark controls that removed all wavelengths of solar radiation. All eggs in the light treatments in the high-UVR lake perished, whereas survival to hatching of eggs in all treatments in the low-UVR lake and in the dark controls in the high-UVR lake were ?96%. Survival time in the high-UVR lake was longer in UV-B-shielded than in fully exposed (quartz) treatments, and eggs collected from the high-UVR lake survived longer than those collected from the low-UVR lake in identical UVR treatments. A survey of natural spawning depths in the two lakes revealed a much deeper spawning depth in the high-UVR lake (median = 3.2 m) than in the low-UVR lake (median = 0.4 m). Deeper spawning depths in the high-UVR lake suggest that yellow perch can avoid direct UVR damage in low- DOC lakes. DOC and hence UVR in lakes may be altered by both anthropogenic and natural disturbances in the surrounding watershed, suggesting that these disturbances may have consequences for the spawning habitat of fish.

Ecological consequences of long-term browning in lakes.

The biological responses of four freshwater daphniid species, Daphnia middendorffiana, D. pulicaria, D. pulex and D. parvula, to a single acute dose of ultraviolet B radiation (UVB) were compared. In addition to survival, we compared the induction of DNA damage (i.e. cyclobutane pyrimidine dimers) between species as well as the ability to repair this damage in the presence or absence of photoreactivating light. All four species showed high levels of shielding against DNA damage when compared to damage induced in purified DNA dosimeters at the same time and dose. Significant variation in survival was observed between species depending on temperature and light conditions. Contrary to our expectations, all species showed significantly higher survival and light‐dependent DNA damage removal rates at 10°C compared to 20°C, suggesting that the enhanced rate of photoenzymatic repair (PER) at the lower temperature contributed significantly to the recovery of these organisms from UVB. PER was highly effective in promoting survival of three of the four species at 10°C, but at 20°C it was only partially effective in two species, and ineffective in two others. None of the species showed significant dark repair at 20°C and only D. pulicaria showed a significant capacity at 10°C. Two species, D. middendorffiana and D. pulex, showed some short‐term survival at 10°C in absence of PER despite their inability to repair any appreciable amount of DNA damage in the dark. All species died rapidly at 20°C in absence of PER, as predicted from complete or near‐absence of nucleotide excision repair (NER). Overall, the protective effects of tissue structure and pigmentation were similar in all Daphnia species tested and greatly mitigated the absorption of UVB by DNA and its damaging effects. Surprisingly, the visibly melanotic D. middendorffiana was not better shielded from DNA damage than the three non‐melanotic species, and in fact suffered the highest damage rates. Melanin content in this species was not temperature dependent under the experimental growth conditions, and so did not contribute to temperature‐dependent responses. It is evident that different species within the same genus have developed diverse biological responses to UVB. Our data strongly suggest that DNA damage is lethal to Daphnia and that photoenzymatic repair is the primary mechanism for removing these lesions. In the absence of light, few species are capable of removing any DNA damage. Surprisingly, the single species in which significant excision repair was detected did so only at reduced temperature. This temperature‐dependence of excision repair is striking and may reflect adaptations of certain organisms to stress in a complex and changing environment.

The paleolimnology of a small waterbody in the Pocono Mountains of Pennsylvania, USA: reconstructing 19th–20th century specific conductivity trends in relation to changing land use

Increases in terrestrially-derived dissolved organic matter (DOM) have led to the browning of inland waters across regions of northeastern North America and Europe. Short-term experimental and comparative studies highlight the important ecological consequences of browning. These range from transparency-induced increases in thermal stratification and oxygen (O2) depletion to changes in pelagic food web structure and alteration of the important role of inland waters in the global carbon cycle. However, multi-decadal studies that document the net ecological consequences of long-term browning are lacking. Here we show that browning over a 27 year period in two lakes of differing transparency resulted in fundamental changes in vertical habitat gradients and food web structure, and that these responses were stronger in the more transparent lake. Surface water temperatures increased by 2–3 °C in both lakes in the absence of any changes in air temperature. Water transparency to ultraviolet (UV) radiation showed a fivefold decrease in the more transparent lake. The primary zooplankton grazers decreased, and in the more transparent lake were largely replaced by a two trophic level zooplankton community. These findings provide new insights into the net effects of the complex and contrasting mechanisms that underlie the ecosystem consequences of browning.

Lipid Dietary Dependencies in Zooplankton

Remains of scaled chrysophytes, magnetic minerals and pollen were used to analyze the recent paleolimnological history of a small lake, Lake Waynewood, in the Pocono Mountains of Pennsylvania. Important shifts in all three variables were observed over the 133 years represented by the core. The most significant changes occurred near the turn of the century when the watershed was heavily logged. Before the logging event, species such asSynura sphagnicola, S. spinosa, Mallomonas galeiformis andM. duerrschmidtiae were co-dominant members of the flora. Subsequent to the deforestation of the watershed other taxa, includingM. crassisquama, M. caudata andS. petersenii, increased in relative importance. Concurrent with changes in the scaled chrysophytes was a six-fold increase in the concentrations of magnetic material, presumably the result of increased erosion caused by the logging. Changes in pollen grains also correlate well with the onset of the deforestation event. The scaled chrysophyte inferred specific conductivity of the lake has more than tripled, with the primary increase occurring concurrent with the commencement of logging and the increase in magnetic mineral material. The effects of other human-related disturbances are also discussed.

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

Zooplankton accumulate large amounts of lipids, as much as 40-70% of the dry mass of their body (Goulden and Henry, 1985; Lee et al., 1972) ome polyunsaturated fatty acids and sterols are essential but required in trace amounts. By contrast, storage and membrane lipids are generally composed of nonessential lipids and make up an important energy reserve.