Temperature limits at the distribution boundaries of four tropical to temperate species of Cladophora (Cladophorales: Chlorophyta) in the North Atlantic ocean

The existence of temperature limits at the geographic distribution boundaries was experimentally examined for four temperate-tropical species of Cladophora (Chlorophyta) in the North Atlantic Ocean; Cladophora vagabunda (L.) van den Hoek, Cladophora dalmatica Kützing, Cladophora albida (Hudson) Kützing and Cladophora ruchingeri (C. Agardh) Kützing. Experimental evidence suggests that all species are prevented from extending beyond their northern boundaries on both sides of the Atlantic Ocean by summer temperatures between 10 and 15°C, which are too low to provide an adequate growing season. The exception was C. dalmatica, which is restricted by lethal winter temperature on north-eastern American coasts. All except C. dalmatica could tolerate some degree of freezing. The lower growth limit was between 10 and 15°C for all species. In accordance with their distribution from tropical to temperate regions, the upper survival limit for three of the species was above 30°C and they grew best at 20–30°C. Cladophora ruchingeri (Ice-landic isolate) was the exception, as it could only survive permanently below 30°C (with optimal growth at 20–25°C) although it is recorded from the tropics, suggesting misidentification or development of ecotypes. Cladophora albida is very rarely recorded from the tropics, but grew and survived better at very high temperatures (30 and 35°C, respectively) than most tropical species. Isolates of C. vagabunda from the cooler waters of Brittany, northern France and the tropical waters of Curaçao differed in their tolerance to low temperatures.