Implications of irradiance exposure and non-photochemical quenching for multi-wavelength (bbe FluoroProbe) fluorometry.

Multi-wavelength fluorometers, such as the bbe FluoroProbe (FP), measure excitation spectra of chlorophyll a (Chl-a) fluorescence to infer the abundance and composition of phytoplankton communities as well as the concentration of chromophoric dissolved organic matter (CDOM). Experiments were conducted on laboratory cultures and on natural communities of freshwater phytoplankton to determine how the response of phytoplankton to high irradiance might affect fluorometric estimates of community composition and concentrations of Chl-a and CDOM. Cultures of a representative cyanobacterium, bacillariophyte, synurophyte, cryptophyte, and chlorophyte revealed changes in Chl-a excitation spectra as irradiance was increased to saturating levels and non-photochemical quenching (NPQ) increased. The degree of change and resulting classification error varied among taxa, being strong for the synurophyte and cryptophyte but minimal for the cyanobacterium. Acute-exposure experiments on phytoplankton communities of varying taxonomic composition from five lakes yielded variable results on apparent community composition. There was a consistent decrease in CDOM estimates, whereas Chl-a estimates were generally increased. Subsequent exposure to low PAR relaxed NPQ and tended to reverse the effects of high irradiance on composition, total Chl-a, and CDOM estimates. Relaxation experiments on near-surface communities in a sixth, large lake, Georgian Bay, showed that total Chl-a estimates increased by 44% on average when dark treatments were used to relax NPQ, though, in contrast to the findings from the small lakes, there was little effect on CDOM estimates. We observed a statistically-significant, negative linear relationship between the photon flux density of in situ irradiance and the accuracy of taxonomic assignment by FP in Georgian Bay. Not discounting the correlations between light intensity and the accuracy of the FP that were observed in this study, we conclude that the applicability of the reference spectra to the system under investigation is a more important consideration than variability in natural irradiance conditions.

Effects of ultraviolet radiation on the productivity and composition of freshwater phytoplankton communities.

The net influence of ultraviolet radiation (UVR; 280-400 nm) on freshwater phytoplankton communities depends on the photon flux density, duration, and spectral quality of exposure and the UVR sensitivity of the assemblage in terms of photosynthetic impairment, biochemical composition, and nutrient assimilation mechanisms. Such effects are mitigated to varying degrees by photoacclimation and selective adaptation at the community level. Variation in UVR penetration among lakes is considerable, largely due to differences in chromophoric dissolved organic matter concentrations. Documented losses of areal daily primary production in lakes due to UVR range from negligible (2.5%) to appreciable (26%). UVR has the potential to alter algal biochemical composition and therefore indirectly affect higher trophic levels. There is evidence that algal nutritional status can influence UVR sensitivity, and that UVR can inhibit uptake and assimilation of inorganic nutrients, but results have been inconsistent. Taxonomic variability in susceptibility to the effects of UVR exists, and likely reflects variation in cell size and shape, concentrations of photoprotective pigments, and capacity to repair UVR photodamage. Suggestions for future research include: (1) resolution of taxon-specific UVR responses by way of single-cell techniques (e.g. enzyme-labelled fluorescence assays, microscope-based variable fluorometers) and (2) systematic comparative studies to link UVR exposure in natural habitats to community responses using the biological weighting function modelling approach. A more robust understanding of how sensitivity to UVR varies according to taxon and habitat is needed if predictions of its role in ecosystem functioning, particularly in connection with climate change, are to be meaningful.

Combined effects of copper and ultraviolet radiation on a microscopic green alga in natural soft lake waters of varying dissolved organic carbon content.

Selenastrum capricornutum was grown in two lake waters of differing dissolved organic carbon content (1.8 vs. 9.1 mg DOCl(-1)) to determine the responses of population dynamics and photosynthesis to Cu, and to assess the modifying effects of varying ultraviolet radiation (UVR) exposure. In the absence of UVR, the mean EC(50) for Cu effect on population growth rate was 2.3-2.6 microg l(-1) in the low DOC water and 17.4-26.2 microg l(-1) in the high DOC water. The variable chlorophyll a fluorescence ratio, F(v)/F(m), decreased approximately in parallel with the diminished growth rates. Exposure of the higher DOC lake water to full spectrum artificial radiation caused an increase of Cu(2+) concentration, compared to samples held in darkness or in photosynthetically active radiation (PAR) only. Full spectrum exposures also resulted in a lower (although not significantly so) EC(50) for Cu effect on growth rate, consistent with response to the moderately elevated Cu(2+) concentration. Cu(2+) concentration was unaffected by radiation exposure in the low DOC water, and EC(50)s for growth were also unaffected except in the most severe UVR treatment, which was >40% inhibited even in the absence of added Cu. Using F(v)/F(m) as an end-point, there was no evidence of interactions between UVR and Cu under the relatively low PAR exposures used here. Algal growth and photosynthesis was extremely sensitive to Cu in these soft lake waters, with EC(50)s close to current water quality standards in the low DOC water.

The ecological effects of naphthenic acids and salts on phytoplankton from the Athabasca oil sands region.

To better elucidate the ecological effects of naphthenic acids and major ions liberated in oil sands development, the summer-time composition of phytoplankton communities in ten water bodies near Fort McMurray (northeastern Alberta) was studied in 1997. The water bodies varied in degree of process water influence, and in age, size and ancillary chemical characteristics. Community biomass of phytoplankton was not systematically related to naphthenic acid or major ion concentrations, even though the higher naphthenate concentrations exceeded published EC50's for acute effects on several different aquatic species. Chlorophyta were frequently dominant, particularly where naphthenate and major ion concentrations were highest. Canonical Correspondence Analysis (CCA) revealed gradients in taxonomic composition at a finer (genus and species) taxonomic level. Despite the simultaneous and uncontrolled variation of other environmental factors, naphthenate and major ion concentrations (as indexed by conductivity) explained a highly-significant 40% of the variation in taxonomic composition. Systems with naphthenates <6.5 mg l(-1) and conductivity <800 PhiS cm(-1) were clustered together near the origin of the CCA plots, suggesting little ecological effect at such concentrations. Taxa associated with elevated naphthenate and/or major ion concentrations were derived from six different algal divisions and included many that were identified as tolerant in previous bioassay experiments. Over the range of concentrations encountered (1.5-45 and 100-3000 mg l(-1) for naphthenates and ions, respectively), CCA indicated that the ecological effect of major ions appeared to be at least as great as that of naphthenates.