Phytoplankton Dynamics in a Changing Environment

Phytoplankton Time-Series in a LTER Site of the Adriatic Sea: Methodological Approach to Decipher Community Structure and Indicative Taxa

“…In fact, the largest Cluster I is indicative of a mixed community composed mainly of nanoflagellates, diatoms and coccolithophores, while the remaining clusters (II, III and IV) represent the period of the year when diatom abundances increase and dominate the community. This alternation between the dominance of different phytoplankton groups is typical of the Gulf of Trieste [5,20] and the wider basin of the northern Adriatic [21,22,42], where nonetheless nanoflagellates contribute up to three thirds of total phytoplankton abundance [40]. The fact that in Cluster I none of the taxa exceeded the threshold pre-set for Xproj is not surprising, since in this large cluster the taxa are close to their expected (average) profile.…”

“…Late winter and early spring were mostly defined by the remnants of the largest cluster in the 4-clustered partition, namely Cluster IX, which in this case consisted of only one indicative species (Meringosphaera mediterranea). The taxa with a higher-than-expected abundance (data not shown in Table 4) were similar to those of Cluster I in the 4-clustered partition, which means that Cluster IX can be interpreted as a mixed community with the dominance of nanoflagellates, which have a known spring peak in the GoT [40]. The fact that this cluster was less present in 2011 and 2012 than in other years seems to agree with the results of Cerino et al, 2019 [5], who described a low nanoflagellate density at the Italian LTER station in those two years.…”

“…Cluster IV that was characterised by the highest diatom diversity was roughly divided into three periods: early spring, summer (mainly July) and autumn. At this level of partitioning, Cluster IV signals the phenology of diatom blooms, which during the study period had two conspicuous peaks, in July and in autumn [17,40]. Interestingly, diatoms at the LTER station in the Italian part of the GoT showed a different phenology in recent years, where between the years 2013 and 2017 the late spring peak of diatoms became the main one during the year, replacing the late winter-early spring bloom of Skeletonema spp.…”

“…A correlation of 0.87 with p-value = 0.001 was obtained between the two ordination indices (Xproj and IndVal). Plankton communities have complex and dynamic structure [40]. To unveil the temporal patterns of these communities it is often necessary to reduce the number of variates (i.e., taxa); however, preserving a representative amount of information is key when choosing different methods of taxa selection.…”

“…In the 4-clustered partition (Figure 7, left), the main distinction that can be made between clusters represent two phytoplankton communities which are known to occur under different environmental conditions in the Gulf of Trieste [40]. In fact, the largest Cluster I is indicative of a mixed community composed mainly of nanoflagellates, diatoms and coccolithophores, while the remaining clusters (II, III and IV) represent the period of the year when diatom abundances increase and dominate the community.…”