Does increased salinity influence the competitive outcome of two producer species?

Abstract

Within the context of global climate changes, it is expected that low-lying coastal freshwater ecosystems will face seawater intrusion with concomitant increase in salinity levels. Increased salinity may provoke disruption of competitive relationships among freshwater species. However, species may be capable of acclimating to salinity, which, in turn, may influence the resilience of ecosystems. Accordingly, this work aimed at assessing the effects of multigenerational exposure to low levels of salinity in the competitive outcome of two species of green microalgae: Raphidocelis subcapitata and Chlorella vulgaris. To attain this, three specific objectives were delineated: (1) compare the toxicity of natural seawater (SW) and NaCl (as a surrogate of SW) to the two microalgae, (2) determine the capacity of the two microalgae species to acclimate to low salinity levels, and (3) assess the influence of exposure to low salinity levels in the competitive outcome of the two microalgae. Results revealed SW to be slightly less toxic than NaCl for the two microalgae. The EC for growth rate was 4.63 and 10.3 mS cm for R. subcapitata and 6.94 and 15.4 mS cm for C. vulgaris, respectively for NaCl and SW. Both algae were capable of acclimating to low levels of salinity, but C. vulgaris seemed to acclimate faster than R. subcapitata. When exposed in competition, under control conditions, the growth rates of C. vulgaris were lower than those of R. subcapitata. However, C. vulgaris was capable of acquiring competitive advantage equaling or surpassing the growth rate of R. subcapitata with the addition of NaCl or SW, respectively. The multigenerational exposure to low levels of salinity influenced the competitive outcome of the two algae both under control and salinity exposure. These results suggest that long-term exposure to low salinity stress can cause shifts in structure of algae communities and, therefore, should not be neglected since algae are at the basis of food web constituting important energetic resources to higher trophic levels.

Sensitivity of freshwater species under single and multigenerational exposure to seawater intrusion

“…The collection site was located in front of a Natural Reserve (São Jacinto dunes, created in 1979) and its waters are subjected to periodic monitoring programmes according to the European Union Directive 2006/7/CE. The waters from this location are considered good to excellent quality and, for the past 28 years, this site has received the prestigious Blue Flag award granted by the Foundation for Environmental Education ( please see [23]).…”

“…Regarding the Chlorophyta species R. subcapitata and Chl. vulgaris, the data generated in Venâncio et al [23] were used to compute the EC 50 values used in this study.…”

“…Many studies suggest that the development of stress mechanisms may be triggered quickly in rapid growth species. For instance, algae can increase their tolerance to salinity within three generations when exposed to sublethal levels of salinity [23]. According to Hart et al [71], the period provided for organisms to acclimate is essential: the longer period organisms have been exposed to a particular salinity level, the higher their ability to later cope with salt stress.…”

“…The majority of studies addressing salinity effects on freshwater biota used mostly specific salts (NaCl, MgCl 2 , among others) in monospecific standard toxicity tests [17][18][19][20][21]. It is still unclear whether such surrogate salts may be safely used for the preliminary risk assessment framework of SW intrusion because available literature comparing toxicity data from surrogate salts and toxicity data of SW is still scarce [10,[22][23][24]. Venâncio et al [24] found that NaCl elicits effects at lower or similar concentrations than natural SW for six clonal lineages of Daphnia longispina.…”

“…through groundwater intrusion) are of relevance, especially for short-lived and multivoltine biota. Therefore, it is important to understand if organisms are capable to acclimate to increased salinity, a phenomenon already reported for algae [23] and daphnids [18,25]. To better cope with salinization episodes, acclimation may occur via short-term compensatory physiological/osmoregulatory changes within their life cycle, like vacuolar compartmentalization or ion sequestration in producers [26], or the triggering of an osmoregulatory reaction in cladocerans [27].…”