Phytogeographic distribution groups of benthic marine algae in the North Atlantic Ocean. A review of experimental evidence from life history studies

Abstract

Experimentally determined lethal temperatures and temperatures limiting growth or reproduction in the life histories of 15 benthic algal species were used to infer possible phytogeographic boundaries in the North Atlantic Ocean. These appeared to correspond closely with phytogeographic boundaries based on distribution data. Many boundaries appeared to be of a composite nature. For instance, the southern boundary ofNemalion helminthoides is interpreted as a “southern reproduction boundary” on the N. Atlantic E. shore and a “southern lethal boundary” on the N. Atlantic W. shore. The northern boundary on both sides of the ocean is a “northern reproduction boundary”.N. helminthoides is a typical representative of the “amphiatlantic temperate distribution group”, to which seven other of the fifteen investigated species belong (Chondrus crispus, Desmarestia aculeata, D. viridis, Monostroma grevillei, Acrosiphonia “arcta” with a comparable composite southern boundary;Rhodochorton purpureum with a “southern lethal boundary”).Polysiphonia ferulacea andDictyota dichotoma are treated as representatives of the “amphiatlantic tropical-to-warm-temperate distribution group”, andP. denudata as representative of the “amphiatlantic tropical-to-temperate group”.P. harveyi belongs to the N.E. American temperate group and is bounded by a “northern reproduction boundary” and a “southern reproduction boundary”. This is one of the very few species endemic to N.E. America. This poor endemism is ascribed to the vast adverse sediment shores and their additional acting as barriers to glacial northsouth displacements of the flora; it is not related to the wide annual temperature fluctuations (>20 °C) typical for N.E. America. The temperate algal flora of Japan, however, which is extremely rich in endemic species is subject to equally wide annual temperature fluctuations.Bonnemaisonia hamifera is such a Japanese endemic, which has been accidentally introduced into the North Atlantic Ocean where its life history seems to be disrupted: it is maintained mainly by vegetative propagation of the heteromorphic tetrasporophyte. The species of the “warm temperate Mediterranean-Atlantic group” are probably too stenothermous for life on N.E. American shores; they need annual temperature fluctuations<20 °C.Acrosymphyton purpuriferum seems to belong to this group, but arguments are presented to unite this species withA. caribaeum and to range it under the “amphiatlantic tropical-to-warm-temperate group”.Clathromorphum circumscriptum belongs to the “Arctic distribution group” and has a “southern reproduction boundary” across the ocean along the 3 °C February isotherm. This species is able to survive temperatures of about 20 °C. Five amphiequatorial temperate species discussed in this paper and four in another related paper have similar maximum winter temperatures of 14–17 °C (mean monthly values) allowing reproduction. Their amphiequatorial distribution can be explained by assuming similar low temperatures in the euphotic zone along E. Pacific and E. Atlantic equatorial coasts i.e. in narrow inshore belts of intensified upwelling during the presumably intensified glacial circulation of the ocean gyres.