Effect of volcanic eruption on nutrients, light, and phytoplankton in oligotrophic lakes

Volcanic eruptions that shape the earth’s surface can have major effect on ecosystems and, as natural experiments, can yield insights into ecological dynamics. On 04 June 2011, a mega-eruption in the Puyehue volcanic complex (Chile) discharged massive amounts of ash and pumice. Using long-term data from five North Andean Patagonian lakes (Espejo, Correntoso, Nahuel Huapi, Gutiérrez, and Mascardi) that received differing levels of ash, we show that, in Lakes Espejo, Correntoso, and Nahuel Huapi, these inputs resulted in 1.5to 8-fold increases in total suspended solids, light extinction, phosphorus concentrations, and phytoplankton biomass relative to pre-eruption conditions. Although ashes affected light scattering, the ultraviolet : photosynthetically active radiation ratio remained , 0.30–0.35 in all the lakes and no changes were seen in dissolved organic carbon in the affected lakes post-eruption. Thus, no differential specific absorption of the different light wavelengths occurred due to ash input. The results of multiple regression analysis identified light extinction coefficient of PAR (KPAR) as the primary variable that was associated with variation in phytoplankton biomass (chlorophyll). Furthermore, incubation experiments demonstrated significant effects of photoinhibition on phytoplankton growth in these lakes at ambient pre-eruption light intensities. Thus, we infer that increased phytoplankton biomass following the eruption likely reflects nutrient (phosphorus) loading and attenuation of excessive light intensities. Volcanic eruptions have shaped much of Earth’s surface over geological time, but they also, in the shorter term, affect ecosystems at local, regional, and even global scales due to ejection and emission of gases, ashes, pumice, and lava. Thus, eruptions present unique opportunities for scientific discovery though such studies are often hindered by a lack of pre-eruption and post-eruption data that allow comprehensive assessment of their effects and the mechanisms of those outcomes (Lindenmayer et al. 2010; Larson 2011). Past studies of the effect of eruptions on aquatic ecosystems have emphasized fertilization by ash-borne elements such as phosphorus and iron (Hamme et al. 2010; Lin et al. 2011). Studies in marine environments have shown that, after volcanic eruptions, the concentrations of chlorophyll, as a proxy of phytoplankton biomass, increase (Hamme et al. 2010; Lin et al. 2011). Paleolimnological evidence from a lake in Iceland also shows that, after a volcanic eruption that deposited considerable amounts of tephra, there was an increase in chlorophyll-derived pigments in sediments, indicating an increase in phytoplankton biomass following volcanic ash deposition (Einarsson et al. 1993). However, increased concentrations of suspended particles in the water column, such as volcano-derived ashes, also increase light scattering and so decrease light penetration (Kirk 1994). In many situations such shading would be expected to negatively affect phytoplankton growth by reducing photosynthesis to levels where it does not exceed respiration (Huisman 1999; Huisman et al. 2002). However, in extremely transparent oligotrophic and ultraoligotrophic aquatic systems, high light intensities in the upper levels of the water column are known to reduce phytoplankton growth because of photoinhibition (Alderkamp et al. 2010; Gerla et al. 2011), consistent with the possibility that, in highly transparent systems at least, increased light attenuation by abiotic particles such as ash may positively affect phytoplankton growth by reducing photoinhibition. Despite widespread recognition that pelagic ecosystem function reflects the joint effects of dynamic light and nutrient supplies modulated by water column physical structure and internal food web interactions (Sterner et al. 1997; Falkowski and Raven 2007), the effect of light has been largely neglected in studies regarding volcanic eruption. The 04 June 2011 explosion of Puyehue-Cordón Caulle (40u359S, 72u079W) in southern Chile (Fig. 1A) provided a unique chance for assessing such dimensions, as the event deposited massive amounts of ash into a set of nearby lakes in Argentine Patagonia that has been extensively studied for 17 yr (Morris et al. 1995; Callieri et al. 2007; Corno et al. 2009). These temperate Andean lakes (located in North-Patagonia around 41uS) are characterized by high transparency and high ultraviolet radiation (UVR) penetration (Morris et al. 1995), where planktonic organisms living in surface waters are chronically exposed to high light intensity and irradiation at damaging wavelengths (Modenutti et al. 2004, 2005). Such an eruption presents not only an opportunity to evaluate how volcanic eruptions affect lakes but also serves as a ‘‘natural experiment’’ to test the roles of nutrient and light in the ecological functioning of large pelagic ecosystems that cannot otherwise be experimentally manipulated. To assess these effects, we documented optical, chemical, and * Corresponding author: [email protected] Limnol. Oceanogr., 58(4), 2013, 1165–1175 E 2013, by the Association for the Sciences of Limnology and Oceanography, Inc. doi:10.4319/lo.2013.58.4.1165