Interactive effects of vertical mixing, nutrients and ultraviolet radiation: in situ photosynthetic responses of phytoplankton from high mountain lakes in Southern Europe

E W Helbling,C Durán,G Herrera,M Villar-Argaiz,J M Medina-Sánchez,V E Villafañe,P Carrillo

doi:10.5194/bg-10-1037-2013

Abstract

Abstract. Global change, together with human activities, has resulted in increasing amounts of organic material (including nutrients) that water bodies receive. This input further attenuates the penetration of solar radiation, leading to the view that opaque lakes are more "protected" from solar ultraviolet radiation (UVR) than clear ones. Vertical mixing, however, complicates this view as cells are exposed to fluctuating radiation regimes, for which the effects have, in general, been neglected. Furthermore, the combined impacts of mixing, together with those of UVR and nutrient inputs are virtually unknown. In this study, we carried out complex in situ experiments in three high mountain lakes of Spain (Lake Enol in the National Park Picos de Europa, Asturias, and lakes Las Yeguas and La Caldera in the National Park Sierra Nevada, Granada), used as model ecosystems to evaluate the joint impact of these climate change variables. The main goal of this study was to address the question of how short-term pulses of nutrient inputs, together with vertical mixing and increased UVR fluxes modify the photosynthetic responses of phytoplankton. The experimentation consisted in all possible combinations of the following treatments: (a) solar radiation: UVR &amp;plus; PAR (280–700 nm) versus PAR (photosynthetically active radiation) alone (400–700 nm); (b) nutrient addition (phosphorus (P) and nitrogen (N)): ambient versus addition (P to reach to a final concentration of 30 μg P L−1, and N to reach N:P molar ratio of 31); and (c) mixing: mixed (one rotation from surface to 3 m depth (speed of 1 m 4 min−1, total of 10 cycles)) versus static. Our findings suggest that under ambient nutrient conditions there is a synergistic effect between vertical mixing and UVR, increasing phytoplankton photosynthetic inhibition and excretion of organic carbon (EOC) from opaque lakes as compared to algae that received constant mean irradiance within the epilimnion. The opposite occurs in clear lakes where antagonistic effects were determined, with mixing partially counteracting the negative effects of UVR. Nutrient input, mimicking atmospheric pulses from Saharan dust, reversed this effect and clear lakes became more inhibited during mixing, while opaque lakes benefited from the fluctuating irradiance regime. These climate change related scenarios of nutrient input and increased mixing, would not only affect photosynthesis and production in lakes, but might also further influence the microbial loop and trophic interactions via enhanced EOC under fluctuating UVR exposure.

Highlights

Increased global temTpehraeturCe,rcyhoansgpeshinerperecipitation, enhanced levels of solar ultraviolet radiation (UVR, 280– 400 nm) together with other anthropogenic activities have significantly altered natural conditions of aquatic bodies, causing direct and indirect effects on phytoplankton (Hallegraeff, 2010; Hader et al, 2011; United Nations Environment Programme, 2012)
This lake is oligotrophic, with low light penetration in the water column (Velasco et al, 1999); the lake watershed area is highly influenced by cattle activity; (b) Las Yeguas (37◦02 N, 3◦22 W), is a small and shallow, with inlets and outlets, and highly transparent oligotrophic lake (Medina-Sanchez et al, 2010) located in Sierra Nevada National Park in the Granada Province; and (c) La Caldera (37◦3 N, 3◦19 W) is a remote high mountain lake located above the tree-line in the Sierra Nevada National Park (Granada Province) with no visible inlets or outlets
This initial condition was obtained during a period without Saharan dust input, nutrients in the water column were low as compared with our treatment with an extra input of nutrients

Summary

Introduction

Increased global temTpehraeturCe,rcyhoansgpeshinerperecipitation, enhanced levels of solar ultraviolet radiation (UVR, 280– 400 nm) together with other anthropogenic activities (e.g. increasing input of nutrients due to land or agricultural use) have significantly altered natural conditions of aquatic bodies, causing direct and indirect effects on phytoplankton (Hallegraeff, 2010; Hader et al, 2011; United Nations Environment Programme, 2012). Under conditions of global warming, it is expected that the temperature increase would produce a stronger (i.e. higher gradient at the thermocline) and shallower epilimnion, which may have profound effects on organisms and ecosystems, especially by exposing them to higher radiation levels and/or a different fluctuating radiation field (Hader et al, 2011) Under this scenario, nutrients would be depleted from the epilimnion (as mixing with deep waters would be prevented) and productivity might decrease (Beardall et al, 2009; Raven et al, 2011); the impacts of UVR might be greater (Litchman et al, 2002; Shelly et al, 2005). Nutrients would not be limiting, as their inputs occur via the atmosphere or carried by rivers, as shown for various lakes and coastal environments (Xenopoulos et al, 2002; Carrillo et al, 2008; Hessen et al, 2008)

Objectives

Methods

Results

Conclusion