Eelgrass, Zostera marina, growth along depth gradients: upper boundaries of the variation as a powerful predictive tool

1200 measurements of eelgrass (Zostera marina) biomass, shoot density and c over along 19 depth gradients in Oresund, located between Denmark and Swede n, were analysed to characterise growth of eelgrass in relation to depth. T he large data set allowed analyses of boundaries of distribution as well as of average trends. Natural variability is large in shallow water where pop ulations are disturbed by wave action and other physical parameters. Models based on average values, therefore, did not adequately describe growth reg ulation by resources, and only explained a minor part (up to 30%) of the ov erall variation in data. In contrast, boundary functions, which describe th e upper bounds of distributions, focus on the variation produced by the ult imately growth-regulating resource, and therefore provide models with high predictive power. An exponential model explained up to 90% of the variation In upper bounds of eelgrass shoot density as a function of depth and indic ated that shoot density was ultimately regulated by light availability. The boundary functions demonstrated that eelgrass shoot density, biomass and c over followed markedly different patterns as functions of depth and were af fected differently by the factors governing their distribution. In addition , boundary functions revealed informative spatial structures in data and il lustrated whether a given general trend was caused by changes in maximum va lues, minimum values or both. For example, upper and lower boundaries of bi omass-shoot density relations changed markedly with depth, demonstrating de pth-related changes in intraspecific succession and competition patterns. B oundary functions are therefore suggested as a promising tool for analysing ultimate regulating factors of distribution and growth of organisms when l arge data sets are available.