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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.