Seaweed functional strategy turnover across intertidal zones: a mathematical model

Abstract The intertidal zone is defined as the area of the coastline exposed at low tide and submerged at high tide. It is characterized by intense environmental gradients of temperature, salinity and water motion, enabling its utilization as a natural laboratory. Although conceptual frameworks have been established, the mathematical modeling of seaweed functional patterns is considered an area of uncertainty. A mathematical model of functional strategy turnover comprising competitive (C), stress-tolerant (S), and ruderal (R) strategies was developed. Vertical transects were sampled on tropical and temperate shores using quadrat point-intercept methods. Seaweed taxa were assigned to functional strategies, and relative frequencies within each quadrat were assessed along a standardized gradient. It was hypothesized that high shore communities dominated by stress-tolerant and ruderal strategists would be replaced by competitive strategists toward the lower shore. This was tested using the ratio of competitive to combined stress-tolerant and ruderal strategists (C/SR ratio). An exponential increase in this ratio from upper to lower zones was observed, by which an exponential turnover in functional composition is indicated. Comparable results were yielded by a finer-scale analysis. A simple, scalable framework for quantifying functional turnover is provided by the proposed C/SR model.