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

Featured article Scientia Marina 84(1) March 2020, 7-16, Barcelona (Spain) ISSN-L: 0214-8358 https://doi.org/10.3989/scimar.04995.06A Photosynthetic activity of macroalgae along a bathymetric gradient: interspecific and seasonal variability Natàlia Sant, Enric Ballesteros Centre d’Estudis Avançats de Blanes, CSIC, Acc. Cala Sant Francesc 14, 17300 Blanes, Girona, Spain. (NS): E-mail: . ORCID iD: http://orcid.org/0000-0002-5660-0131 (EB) (Corresponding author) E-mail: . ORCID iD: http://orcid.org/0000-0001-5532-5337 Summary: 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. Keywords: macroalgae; photosynthesis; depth gradients; efficiency; light. Actividad fotosintética de macroalgas a lo largo de un gradiente batimétrico: variabilidad interespecífica y estacional Resumen: Las macroalgas marinas se distribuyen de forma desigual en profundidad, observándose una substitución de las especies dominantes a lo largo del eje batimétrico. Como la luz muestra una disminución muy marcada con la profundidad y es un factor crítico para el crecimiento de las algas, nuestra hipótesis es que aquellas especies que dominan a una cierta profundidad son las que están mejor adaptadas al ambiente lumínico de esa profundidad. Para comprobarlo hemos seleccionado siete especies de algas marinas mediterráneas distribuidas entre 0 y 50 m de profundidad y hemos calculado sus parámetros fotosintéticos. Las respuestas fotosintéticas muestran grandes diferencias entre especies, con las eficiencias fotosintéticas que aumentan al aumentar la profundidad donde crecen las algas. Sin embargo, tanto la producción de oxígeno a elevados niveles de luz como la respiración no están relacionadas con la profundidad, y parecen determinadas por otros factores como el grosor del talo o la disponibilidad de nutrientes. Algunas especies muestran valores de producción elevados, en sintonía con su alta productividad y rápido crecimiento, mientras que otras muestran bajos valores de producción pero una elevada biomasa y perdurabilidad. Así pues, las especies que dominan en cada ambiente lumínico parecen ser aquellas que utilizan de mejor manera la luz disponible donde crecen para completar con éxito su ciclo de vida. Palabras clave: macroalgas; fotosíntesis; gradientes de profundidad; eficiencia; luz. Citation/Como citar este artículo: Sant N., Ballesteros E. 2020. Photosynthetic activity of macroalgae along a bathymetric gradient: interspecific and seasonal variability. Sci. Mar. 84(1): 7-16. https://doi.org/10.3989/scimar.04995.06A Editor: E. Cebrián. Received: September 12, 2019. Accepted: January 21, 2020. Published: February 14, 2020. Copyright: © 2020 CSIC. This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0) License. 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). Light is also a resource that is critical for algal growth and it is the main limiting factor for deep-water algal development in places where appropriate substrate is available (Spence 1976, Denni- 8 • N. Sant and E. Ballesteros son 1987, Lobban and Harrison 1994). 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. However, the maximum depth where a macrophyte can grow also depends greatly on its physiological features. 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. As a result, different species of aquatic macrophytes usually dominate at different depth intervals, a feature that is known as vertical zonation (Dring 1986, Kirk 1994). The presence/absence of a macrophyte at a certain depth depends on the equilibrium between its physiological ability to grow (which in turn depends on the available light, temperature, nutrient availability and other factors) and the ecological processes that imply biomass loss (including unsuitable environmental conditions and herbivory), together with the ability to outcompete with or tolerate other species (Airoldi 2000). In this context, we hypothesize that dominant species at a certain depth are those that outcompete others, whether because they can grow faster or because they are persistent in occupying space. Since light is a pivotal environmental factor for macrophyte growth and it decreases with depth, we expect macrophytes that dominate at different depths to show different photosynthetic performances related to light availability. In order to provide species distribution models, it is necessary to know the physiological responses of organisms to changing environments. In terrestrial ecology one of the most accepted findings is the existence of differences in the photosynthetic performance of “sun plants” versus “shade plants” (Boardman 1977, Field 1988, Boston et al. 1989): plants living in high light environments should display a low photosynthetic efficiency at low light levels but a high photosynthetic capacity at high light levels, while those living at low light environments should display a high photosynthetic efficiency at low light levels and low photosynthetic capacity at high light levels. In benthic ecology, even if this model has been tested experimentally (Herbert and Waaland 1988, Delgado et al. 1995, Beach and Smith 1996), some studies demonstrate that the photosynthetic capacity is mainly related to thallus morphology and thickness (...truncated)