TEMPERATURE LIMITS AT THE DISTRIBUTION BOUNDARIES OF EUROPEAN AND AMERICAN WARM TEMPERATE CLADOPHORA SPECIES (CLADOPHORALES, CHLOROPHYTA) IN THE NORTH-ATLANTIC OCEAN

The existence of temperature limits at the distribution boundaries is examined experimentally for three species of Cladophora in the North Atlantic Ocean: C. pellucida (Hudson) K tzing and C. hutchinsiae (Dillworth) K tzing, found only on European warm temperate coasts, and C. pellucidoidea van den Hoek from warm temperate (to tropical) American coasts. The two European species grew well from 15 to 25 �C, consistant with their distribution in warm temperate regions. Growth and reproduction were limited at and below 10 - 15 �C, indicating that their northern boundaries on open Atlantic coasts are determined by summer temperatures too low for sufficient growth. Their lower survival limit was between 0 � 5 �C, so lethal winter temperatures will restrict their distribution into the North Sea. Their upper survival limit was close to 30 �C. Lethal summer temperatures at C. hutchinsiae ’s southern boundary in the Mediterranean restrict the species distribution further into the warmest waters. Cladophora pellucida had slightly better growth and survival potential at 30 �C which corresponds with its reported occurrence through the warmest waters of the eastern Mediterranean. An isolate of C. pellucida from the cooler waters of Brittany, N. France was found to be slightly more cold tolerant than an isolate tested from the warmer waters of Corsica in the Mediterranean. The American endemic C. pellucidoidea grew well from 20 to 30 �C, consistent with its distribution in the tropical Caribbean but it did not survive permanently at 35 �C. It was less cold tolerant than the European species, with a lower survival limit between 5 and 10 �C, so that lethal low winter temperatures will restrict its northward distribution beyond N. Carolina. Copyright © 1990 Walter de Gruyter