Text-figs. 1-7)The definition of Ectocarpus by Hamel (1931-39) is accepted and his division of the genus into two species complexes, E. fasciculati and E. siliculosi, vindicated. Observations on the E. fasciculatus complex show that its component taxa represent various stages in thallus growth and elaboration, while the subspecific taxa of E. siliculosus are determined primarily by size and shape of plurilocular sporangia. Both species are shown to undergo a streblonemoid phase in their early development. Field observations are confirmed by following the growth of both species in culture. The habitat and host range of both species is found to be wide.The occurrence of many intermediate plants between the subspecific taxa in each species is recorded; so, too, is the occasional occurrence of intermediate plants between E. fasciculatus and E. siliculosus. A simple quantitative assessment of certain taxonomic characters is made and from this and other evidence it is concluded that there is a taxonomic cline within each species such that retention of subspecific taxa becomes impracticable. There is also a cline between both species but this is less marked and the two remain fairly clearly discernible.It is concluded that E. erectus Kiitz. is probably only a protracted juvenile stage of E. siliculosus.

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A decrease in salinity and temperature over the past 3000 years has presented the marine algae of the Baltic Sea with very considerable problems in adaptation. The effects of salinity upon a number of Baltic algae have been measured. The results showed cell mortality to be severe in 0, 68 and 102%o, and minimal in 6 and 11%o: there was most variation in tolerance to 34 and 51°/oo . The salt tolerances of Baltic marine algae have proved more hyposaline than those of British intertidal algae. Water uptake and loss in tissues of Chorda filum and Fucus vesiculosus from Baltic and British populations have been measured in response to salinity changes. The results revealed significant population differences in both live and killed tissues. Receptacle development and oogonial maturation have been observed in Baltic and British F. vesiculosus, and found to differ in seasonality. Some observations were associated with local sea temperatures but differences in the timing of receptacle initiation and in oogonial size were not. The depauperate thallus, commonly ascribed to the effects of low salinity, was found to be a complicated phenomenon, comprising numerous attributes which are combined differently in different taxa. The morphological differences between Baltic and British marine algae were usually striking.

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Abstract. The redevelopment of disused docks around Liverpool into areas suitable for shops, offices, housing and recreational watersports has necessitated the maintenance of good water quality in these areas. The main source of water to the docks is the River Mersey, which contains high concentrations of nutrients, encouraging algal blooms ‐ including potentially nuisance species ‐ in the enclosed dock basins. Since 1988 an extensive monitoring programme has been run sampling the hydrography, plankton and benthos of the docks. During this period there has been an improvement in water quality, including a reduction in summer phytoplankton biomass and anoxic bottom water. There are strong indications that phytoplankton biomass has been severely reduced by the filter feeding of Mytifus edulis. Changes in the composition of the benthic community since 1988 suggest that the large population of M. edulis is now suffering from limited recruitment. The long‐term development and stability of the system is discussed with reference to community succession and its link to ecosystem functioning.