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Cladophora glomerata (L.) Kütz. is, potentially, the most widely distributed macroalga throughout the world's freshwater ecosystems. C. glomerata has been described throughout North America, Europe, the Atlantic Islands, the Caribbean Islands, Asia, Africa, Australia and New Zealand, and the Pacific Islands. Cladophora blooms were a common feature of the lower North American Great Lakes (Erie, Michigan, Ontario) from the 1950s through the early 1980s and were largely eradicated through the implementation of a multibillion-dollar phosphorus (P) abatement program. The return of widespread blooms in these lakes since the mid-1990s, however, was not associated with increases in P loading. Instead, current evidence indicates that the resurgence in blooms was directly related to ecosystem level changes in substratum availability, water clarity, and P recycling associated with the establishment of dense colonies of invasive dreissenid mussels. These results support the hypothesis that dreissenid mussel invasions may induce dramatic shifts in energy and nutrient flow from pelagic zones to the benthic zone.

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We calibrated a model for ultraviolet radiation (UVR) and photosynthetically active radiation-dependent photoinhibition, with explicit damage and recovery processes (the R model), against observations of photosynthesis by Lake Erie phytoplankton exposed to natural sunlight in the summer of 1998. The model explained 74%-96% of the variation in photosynthetic rates and indicated active recovery processes at 4 of the 5 experimental stations. UVR-dependent photoinhibition kinetics were not as well explained by two simpler models that assumed either a lack of recovery processes or an instantaneous equilibrium between damage and recovery. The 1998-calibrated R model also provided consistently significant predictions of photoinhibition in 10 experiments done the previous year, albeit with a reduced goodness-of-fit, whereas simpler models did not. Biological weighting functions (BWFs) derived for UVR effects in 1998 were similar in shape throughout the UVR part of the spectrum, with an especially steep decrease from 300 to 320 nm, compared with BWFs published for other phytoplankton communities and/or species. The R model predicted photoinhibitory losses of primary production, integrated through the photic zone, that were intermediate between the two simpler models and showed that Lake Erie phytoplankton varied in both their spectral sensitivity (as expressed by the BWF) and recovery rates.

We used a spectrally resolved kinetic model to calculate ultraviolet radiation (UVR) and photosynthetic active radiation (PAR)-dependent photoinhibitory losses in planktonic primary production in a large lake (Lake Erie) under mixing and water transparency scenarios typical of current and possible future environmental conditions. The model, previously calibrated for Lake Erie phytoplankton, also provided estimates of photoinhibition recovery rates under high irradiance conditions that were compared against direct measurements of recovery rates under lower irradiance. Extensive recovery of photosynthesis, even after severe (80%) inhibition, occurred after transfer to benign, low irradiance conditions. Measured recovery rate constants were independent of preexposure treatment (median of 1.70 ϫ 10 Ϫ4 s Ϫ1 ) and were comparable to modeled rates under higher radiation fluxes (2.10 ϫ 10 Ϫ4 s Ϫ1 ). Recovery rates were sufficient to allow near-full recovery within one photoperiod, even after severe inhibition (Ͻ6 h). Estimates of the photoinhibitory loss of primary production, integrated through the mixed layer, were not greatly affected by mixing rate variations but were higher in all scenarios with finite mixing rates than in those with no mixing. Modeled 20% stratospheric ozone reductions resulted in small increases in integrated UVR photoinhibition (Ͻ1%), as did 20% and 50% changes in dissolved organic carbon (DOC) concentration, whereas increased maximum water clarity scenarios decreased integrated photoinhibition estimates. While UVA, not UVB or PAR, caused most of the photoinhibition in Lake Erie phytoplankton, the extent of integrated photoinhibition is likely to depend mostly on algal physiological parameters of UVR susceptibility (sensitivity and recovery) and the ratio of mixing depth to PAR and UVR photic depths.The main targets of ultraviolet radiation (UVR) in phytoplankton are DNA and photosystem II structural proteins (D1), so recovery from exposure likely involves both the repair of damaged DNA and the de novo synthesis of chloroplast proteins. Some of the enzymes involved in repairing damaged DNA are inducible. The transcription of photolyase is under light control and is inducible by UVB in plants, and DNA polymerase and DNA ligase are also inducible (Pang and Hays 1991;Greenberg et al. 1997). The control of transcription of these different enzymes is under regulatory control by kinases that are involved in signal transduction and are themselves inducible by growth arrest and DNA damage (Pang and Hays 1991). Evidence for the necessity of de novo chloroplast protein synthesis for recovery of photosynthetic activity comes from experiments, specifically those that use streptomycin to inhibit chloroplast protein synthesis (Lesser et al. 1994;Ihle 1997). In the presence of this protein synthesis inhibitor, recovery does not occur.Recovery of photosynthesis and related processes from UVR-dependent photoinhibition may require only few hours or Ͼ20 h (e.g., Larkum and Wood 1993; Hermann et al.

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