Fifty cyanobacterial strains (10 genera) were tested in batch culture for their ability to use organic phosphorus compounds (1 mg liter(-1) P) as their sole P source. Two monoesters, Na2-β-glycerophosphate and π-nitrophenyl phosphate (πNPP), supported growth of all strains, and the diester bis-π-nitrophenyl phosphate (bis-π-NPP) and herring sperm DNA supported almost all strains. ATP was either a very favorable or poor P source and failed to support growth of nine strains, seven of which were Rivulariaceae with trichomes ending in a hair or long tapered region. Phytic acid was in general the least favorable P source.P-limited cultures grown initially with inorganic phosphate to conditions of P limitation were also tested for cell-bound and extracellular phosphomonoesterase (PMEase) and phosphodiesterase (PDEase) activities at two pH values (7.6, 10.3) using πNPP and bis-πNPP as substrates. Cell-bound PMEase was inducible in all strains and cell-bound PDEase in most strains. Most showed extracellular PMEase, but not extracellular PDEase. The highest values (μM πNPP or bis-πNPP hydrolyzed mg dry weight(-1) hour(-1)) all occurred in strains ofGloeotrichia as follows: cell-bound PMEase at pH 7.6, 2.7 μM in strain D602; cell-bound PMEase at pH 10.3, 5.2 μM in D602; extracellular PMEase at pH 7.6, 0.73 μM in D281; extracellular PMEase at pH 10.3, 6.6 μM in D281; cell-bound PDEase at 7.6, 0.40 μM in D613; cell-bound PDEase at pH 10.3, 1.0 μM in D613.The results were compared to see if they indicated possible relationships between phosphatase activity and taxonomic or ecological grouping. The following differences were significant (P<0.05). Rivulariaceae produced higher yields than filamentous non-Rivulariaceae with β-glycerophosphate, πNPP, and DNA. Rivulariaceae with the ability to form hairs in culture showed poorer growth in ATP than non-hair-forming Rivulariaceae, but were more effective at utilizing phytic acid. Strains from calcareous environments had higher PMEase activity at pH 10.3 than strains from noncalcareous environments (P<0.01).

Key features of photosynthetic picoplankton populations were compared during 1988 in ten lakes in northem England ranging from oligotrophic to slightly eutrophic; two of the three eutrophic lakes were shallow and lacked a thermocline. Measurements were made at 0.5 m depth of temperature, total chlorophyll a, chlorophyll-containing picoplankton cell density, mean picoplankton Celt volume and percentage of phycoerythrin-rich cells in the total picoplankton population. All lakes showed maxima for total chlorophyll concentration and picoplankton cell density in mid-to late summer. The maximum value for picoplankton density ranged from 3.4 x 103 (Esthwaite Water) to 1.3 x 106 cells ml-1 (Ennerdale Water).There was a significant negative relationship (p c 0.05) between log,,, of maximum picoplankton cell density and maximum total chlorophyll, the latter being taken as an indicator of lake trophic status. The ratio of maximum to minimum picoplankton density during the year in a particular lake ranged from 39 to 2360 and showed no obvious relationship to lake type. Overall, the seasonal range in picoplankton density was about one order of magnitude greater than the range in total chlorophyll a, but there were considerable differences between lakes.Phycoerythrin-rich picoplankton as a percentage of total picoplankton reached a maximum in summer in all lakes. Values were always very low (c 5 %) in the two shallow eutrophic lakes, but reached 97 % and over in the four most oligotrophic lakes. In two of the oligotrophic lakes, Wastwater and Ennerdale Water, phycoerythrin-rich picoplankton was a major component of the summer phytoplankton biomass.

An extensive bloom of Hydrodictyon reticulatum was observed on Loe Pool, a eutrophic coastal freshwater lake in Cornwall. In previous years the Pool has experienced cyanobacterial blooms, but in 1993 Hydrodictyon became dominant for the first time. It was estimated that approximately 24% of the Pool was covered by Hydrodictyon and dissolved oxygen saturations were observed to range between 220% and 34% within a mat of Hydrodictyon. The reason for the shift in algal dominance remains unclear, though large numbers of zooplankton were observed sheltering around the stands of Hydrodictyon. These may have restricted the growth of phytoplankton once the Hydrodictyon had become established. It is concluded that further study is required to determine the causal mechanism which may be responsible for the explosion in Hydrodictyon. The possibility of the growth of a more vigorous strain cannot be discounted.

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