Photosynthetic activity of macroalgae along a bathymetric gradient: interspecific and seasonal variability

Natàlia Sant,Enric Ballesteros

doi:10.3989/scimar.04995.06a

Abstract

Marine macroalgae are unevenly distributed with depth, with substitutions of the dominant species along the bathymetric gradient. Since light shows a sharp decrease with depth and is a critical factor for algal growth, we hypothesize that species dominating at a certain depth are the best adapted to the light environment reaching that depth. To this end, we selected seven species of Mediterranean macroalgae distributed from 0 to 50 m depth and looked for their photosynthetic performances. Photosynthetic responses showed striking differences, with photosynthetic efficiencies increasing at increasing depths. However, neither oxygen production at high light levels nor respiration are related to depth but seem to respond to other factors such as thallus thickness or nutrient availability. Some species show high production values in agreement with their high productivity and fast growth, while others show low production values but high biomass and durability. Thus, the species that dominates at each light level seems to be the one that makes best use of the light environment to accomplish its life cycle.

Highlights

Bathymetric distribution of aquatic macrophytes depends on several environmental factors, including the type, slope and orientation of the substrate, light intensity, hydrodynamism, nutrient availability and grazing pressure
Light levels and hydrodynamism show a sharp decrease with depth (Ballesteros and Zabala 1993)
The southern part of the Estell des Coll islet has an almost vertical wall that sinks down to 52 m depth, showing five different algal assemblages (Ballesteros et al 1993, Sant et al 2017) from the shallows to the bottom: the assemblages dominated by Carpodesmia amentacea v. stricta (0.2 m depth), Padina pavonica and Dictyota fasciola (1-8 m depth), Carpodesmia brachycarpa v. balearica (10-16 m depth), Dictyopteris polypodioides (18-40 m depth) and the coralligenous assemblage (42-52 m depth) dominated by calcareous encrusting algae (Ballesteros 2006), with Flabellia petiolata and Halopteris filicina as the main erect species at the sampling site

Summary

Introduction

Bathymetric distribution of aquatic macrophytes depends on several environmental factors, including the type, slope and orientation of the substrate, light intensity, hydrodynamism, nutrient availability and grazing pressure. Among these factors, light levels and hydrodynamism show a sharp decrease with depth (Ballesteros and Zabala 1993). The maximum depth of most aquatic macrophytes in a specific location is inversely related to the capacity of water to absorb the photosynthetic active radiation (Kirk 1994), i.e. the clearer the water is, the deeper the algae can grow. Some macrophytes (photophilic) are always found in shallow waters, where they are subjected to strong light levels, while others (sciaphilic) mainly grow in deep waters or in places with low light levels

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