Phytoplankton functional traits and seston stable isotopes signature: a functional-based approach in a deep, subalpine lake, Lake Maggiore (N. Italy)

Abstract

The seasonal variation of seston stable isotopes signature of carbon (δ13C) in Lake Maggiore during 2008 was related to seasonal variation of the lake phytoplankton community, investigated in terms of phytoplankton taxonomic groups, morpho-functional groups (MBFG), cell size classes and cell shape classes. Three open water stations were selected to reflect truly pelagic, influenced by littoral and riverine carbon sources; phytoplankton samples were collected from two water depths, 0-20 m and 25-50 m. Among stations differences in δ13C signatures of seston were statistically non significant, confirming that allochthonous input may become important only after exceptional rainfall events. Nonparametric multiplicative regression (NPMR) was utilized to identify among the phytoplankton parameters (taxonomic groups, MBFG, cell shapes, cell sizes) which were the best predictors of the δ13C variation. Bacillariophyceae and two morpho-functional groups, Group 6 (non-flagellated organisms with siliceous exoskeletons) and Group 3 (large filamentous algae with aerotopes) were statistically significant. Non-metric multidimensional scaling (NMS) ordination was used to investigate differences in phytoplankton samples and their relationship with δ13C variation. When morphological and functional phytoplankton traits were superimposed, the NMS ordination showed that cylinder cells (for cell shape classes), class 500-2000 mm3 (for cell size classes) and Group 6 (for MBFG) were the most related to δ13C variation. Our study confirms that the variation in the pelagic δ13C seston most probably reflects changes in phytoplankton carbon isotopic signature, consequent to changes in availability of carbon sources depending on the season and due to different isotopic fractionation of phytoplankton taxonomic groups. Statistical investigations have allowed us to investigate the potential role of phytoplankton morphological and functional traits in the seasonal variation of δ13C and to suggest their importance in the interpretation of natural stable isotopes studies in lake ecosystems

Defining Seasonal Functional Traits of a Freshwater Zooplankton Community Using δ13C and δ15N Stable Isotope Analysis

“…In Lake Maggiore, Bacillariophycee are the most dominant phytoplankton group with a high biomass throughout the year [37,40]. In the year of our study (2009), a peak in the Bacillariophycee biomass of Aulacoseira sp., Asterionella sp., and Fragilaria sp.…”

“…Seasonal changes in the littoral and pelagic δ 13 C isotopic baseline were identified by a temporal shift towards less 13 C-depleted values in the summer, which is a trend commonly observed in thermally-stratified lakes [35,36]. In lacustrine systems, δ 13 C and δ 15 N of suspended particulate matter (seston) varies seasonally [37] because of differences in the allochthonous input, phytoplankton species composition, and primary productivity [38,39]. In Lake Maggiore, it has been demonstrated that δ 13 C of the primary consumer Daphnia longispina galeata gr.…”

“…In the year of our study (2009), a peak in the Bacillariophycee biomass of Aulacoseira sp., Asterionella sp., and Fragilaria sp. was recorded in spring [37], when the cladoceran Daphnia longispina galeata gr. had the most δ 13 C-depleted values, while when Cyanobacteria biomass increased in the lake during the summer and autumn, Daphnia longispina galeata gr.…”

“…If we consider the phytoplankton component of seston, there is variation in the fractionation of δ 13 C between phytoplankton groups, with Chrysophycee and Bacillariophycee being more depleted in δ 13 C than Cyanobacteria [37]. In Lake Maggiore, Bacillariophycee are the most dominant phytoplankton group with a high biomass throughout the year [37,40].…”

Changing ecology of Lake Victoria cichlids and their environment: evidence from C13 and N15 analyses

“…N values of primary producers and primary consumers have been reported during periods of stratification in temperate lakes, but to our knowledge not in tropical lakes (Quay et al, 1986;Zohary et al, 1994;Hodell & Schelske 1998;Caroni et al, 2012). In addition, larger consumers such as fish have long tissue turnover rates (months to years, Hesslein et al, 1993) and thus are their isotopic signatures representative of their diet for longer periods of time (Post, 2002).…”