Time-constrained sampling: a little-explored alternative for marine hard bottom communities

Marine hard bottoms support the highest proportion of marine biodiversity. In addition, there are a wide range of sizes and forms among the benthic species of a hard bottom community: solitary or modular, from small organisms to large erected colonies, epibionts, encrusting fauna, mobile predators, etc. These habitats and their biological communities have a high heterogeneity at different spatio-temporal scales. The high diversity and heterogeneity of the hard bottom habitats can present considerable difficulties to obtain accurate and statistically comparable data for community monitoring. Thus, marine hard bottoms are complex habitats where monitoring protocols vary considerably. We review the main approaches used in the study of this type of habitat, assessing some of their main advantages and disadvantages, and highlight a not sufficiently explored alternative where the sampling effort is controlled by a set period of sampling time. Time-constrained searches have been routinely used in terrestrial ecological studies but few explore in marine habitats. However, it is an acceptable alternative that would be worth further exploring for studding marine hard bottom communities. Time-constrained methodology presents a good cost-benefit balance: It provides a good representation of the diversity of hard bottom communities, the costs (both in time and economic resources) for sampling and sorting are lower, and the quantification of taxa allows their relative abundance to influence the results.