Lntertidal Macrophyte Comrnunities from Pacific Baja California and the Upper Gulf of California Relatively Constant vs. Environmentally Fluctuating Systems

Abstract

The adaptive features of marine macrophyte communities in a markedly fluctuating environment (the northern Gulf of California) were contrasted with those characteristic of a climatically more constant system (the Pacific coast of the Baja Peninsula) in the light of a priori predictions. In general, the data suggest the importance of intermediate disturbance in permitting a mosaic of young to mature community types to CO-occur in accordance with previous theories. In the less-disturbed (more constant) Pacific coast system, the predominant macrophytes were either calcareous algae or structurally complex vascular plants, with early successional species such as Ulva californica and Gelidium pusillum being abundant only on recently overturned boulders. The majority of the overall productivity on the Pacific coast was provided by the larger, most abundant species. However, production rates per unit of thallus were highest for small species with high surface area to volume ratios (i. e., Navicula sp., U. californica. Chondria californica) that grew on the overturned rocks. As a group, the macrophytes that dominated the standing stock had considerably lower net productivity and larger thalli which were geometrically complex and formed a layered community that increased spatial heterogeneity. The Pacific site was more diverse (Shannon-Weaver H') , having higher values for both richness (species numbers) and evenness (J'). Conversely, in the seasonably disturbed sites (due to catastrophic environmental fluctuat~ons) annual or opportunistic forms with high reproductive output (e. g.. U. rigida, various species of Colpomenia, as well as eurytopic blue-green algae that are indicative of harsh habitats) were significantly more abundant (P < 0.05, Mann-Whitney U-test) than at the Pacific site. These populations rapidly recolonized available substrate following periods of mortality caused by extreme heat, desiccation and insolation during summer low tides. The smaller turf-like growth habits of the dominant macrophytes in the Gulf communities provided little spatial heterogeneity and this appeared to further limit macrobiotic diversity. Forms that contained considerable structural tissue or calcium carbonate as part of their thallus weight were the lowest producers at the Gulf sites and were of moderate to low abundance. Selection in the seasonally disturbed environments appears to have favored a predominance of structurally simple, opportunistic species (e. g., Ulva, Colpomenia) having high individual net productivity. In the less disturbed Pacific system, photosynthetic rates of the large predominant species (e. g., calcareous forms, thick forms, prostrate crusts, morphologically differentiated flowering plants) were lower because they have evolved structurally complex thalli (at the expense of photosynthetic tissue) in order to withstand the greater pressures of biological interactions. Most of the populational and community features at the Pacific site suggest a relatively longlived, mature intertidal system containing a mosaic of younger successional stages; whereas, environmental fluctuations beyond the homeostatic capabilities of most marine plants maintain the upper Gulf macrophyte communities in a simpler subclimax condition.