Silvia Pulina

Climate Change and Eutrophication Induced Shifts in Northern Summer Plankton Communities

Marine ecosystems are undergoing substantial changes due to human-induced pressures. Analysis of long-term data series is a valuable tool for understanding naturally and anthropogenically induced changes in plankton communities. In the present study, seasonal monitoring data were collected in three sub-basins of the northern Baltic Sea between 1979 and 2011 and statistically analysed for trends and interactions between surface water hydrography, inorganic nutrient concentrations and phyto- and zooplankton community composition. The most conspicuous hydrographic change was a significant increase in late summer surface water temperatures over the study period. In addition, salinity decreased and dissolved inorganic nutrient concentrations increased in some basins. Based on redundancy analysis (RDA), warming was the key environmental factor explaining the observed changes in plankton communities: the general increase in total phytoplankton biomass, Cyanophyceae, Prymnesiophyceae and Chrysophyceae, and decrease in Cryptophyceae throughout the study area, as well as increase in rotifers and decrease in total zooplankton, cladoceran and copepod abundances in some basins. We conclude that the plankton communities in the Baltic Sea have shifted towards a food web structure with smaller sized organisms, leading to decreased energy available for grazing zooplankton and planktivorous fish. The shift is most probably due to complex interactions between warming, eutrophication and increased top-down pressure due to overexploitation of resources, and the resulting trophic cascades.

Long-term phytoplankton dynamics in a Mediterranean eutrophic lagoon (Cabras Lagoon, Italy)

Abstract The temporal variations in phytoplankton abundance and diversity were analysed from 1999 to 2009 in Cabras Lagoon, a shallow eutrophic Mediterranean lagoon (west coast of Sardinia). It is one of the aquatic ecosystems listed in the “Marine Ecosystems of Sardinia” by the Italian Network of Long-Term Ecological Research (LTER Italy). The objectives of this work were to assess the general features of phytoplankton succession and its dynamics in relation to environmental factors along the decade 1999–2009, and to identify the main forces driving phytoplankton variability in terms of the general trends in Mediterranean transitional waters. Our results highlight several important aspects: a shift in the annual peak density from the summer–autumn season, observed in 1999–2002, to the winter–spring season in subsequent years; simultaneous reductions in chlorophyll a concentrations and cell size from 1999 to 2008; and the modification of the structure of the phytoplankton assemblages, from a more differentiated class composition in 1999–2002 to the subsequent dominance of the Cyanophyceae. Moreover, analysis of similarity on both abiotic and phytoplanktonic data shows significant differences among the hydrological years considered. Pearsons correlations indicate temperature, salinity and nutrients as the environmental parameters that most influenced the phytoplankton composition and dynamics.

Phytoplankton dynamics in relation to environmental changes in a phytoplankton-dominated Mediterranean lagoon (Cabras Lagoon, Italy)

In Mediterranean lagoons, macrophytes often surpass phytoplankton as the most important primary producers. Less frequently, phytoplankton dominates throughout the year, thus knowledge of its dynamics is relatively limited and scattered. In this study, we assessed over two years the dynamics of phytoplankton assemblages, including potential harmful algal species (HAS), in relation to environmental changes in the phytoplankton-dominated Cabras Lagoon (Sardinia, Italy). The lagoon was characterised by uniform spatial conditions, wide temporal variations in salinity (40 PSU) and high nutrient availability. Phosphorus was highest in summer, possibly recycled within the system, while dissolved inorganic nitrogen increased in winter and spring due to watershed discharge. Chlorophyll a, positively correlated with nutrients and rainfall, showed a typical bimodal pattern with summer-winter blooms. Modifications in phytoplankton composition strongly correlated with extreme weather events, such as intense rainfall. This generated an abrupt salinity decrease that, combined with high nutrient availability, favoured the dominance of Cyanophyceae of reduced cell size, such as Cyanobium and Rhabdoderma species. We suggest that the prolonged and intense dominance of Cyanophyceae, added to other HAS, has a negative impact on the primary economic activities of the lagoon, such as fishery, and generally on the whole lagoon functioning.

Long-term ecological studies on phytoplankton in Mediterranean reservoirs: a case study from Sardinia (Italy)

Abstract Artificial lakes are strategic water resources in the Mediterranean region but are one of the most vulnerable areas to the impacts of climate change, particularly to the expected significant decrease in water resources; therefore, enhancing the ability of water management to improve water quality is crucial. This study contributes to the knowledge of the long-term ecological features of Mediterranean artificial lakes based on a case study of Bidighinzu Lake in Sardinia, Italy, a warm, monomitic and hypereutrophic reservoir mainly used for drinking water. Among the different restoration actions carried out in Bidighinzu Lake to improve water quality is the reduction of nutrient loads from the watershed. To study multiannual trends of the trophic state, a long-term series (1988–2012) of both environmental variables and phytoplankton was analysed. Mann-Kendall tests revealed increasing trends for total nitrogen to total phosphorus ratios (TN:TP; significant in spring, summer, and winter) and decreasing trends for ammonium (winter and autumn), TP (all seasons), orthophosphate (spring, summer, and winter), reactive silica (autumn), and pH (summer). Among phytoplankton, significant increasing trends were assessed in biomass of Bacillariophyceae, Chlorophyceae, Dinophyceae, and Chrysophyceae. Cyanobacteria did not show a clear trend, although at the order level, Chroococcales significantly increased in summer and decreased in winter. Despite these major tendencies, the lake did not show significant improvement in its eutrophic state, probably due to an inherent resistance and the insufficient and/or not well calibrated or applied recovery actions.

Multiannual phytoplankton trends in relation to environmental changes across aquatic domains: A case study from Sardinia (Mediterranean Sea)

We investigated multiannual trends in phytoplankton in relation to several environmental drivers. We analyzed ecological data collected during the past two decades from three aquatic sites: an artificial lake, a coastal lagoon, and a marine coastal area. Hydrographic, nutrient, and phytoplankton data were statistically analyzed to detect trends and interactions. In all ecosystems, the chlorophyll a concentration decreased with increasing abundance of small-sized phytoplankton. Phytoplankton dynamics were related to decreased nutrient concentrations in the lake, to dynamics of phosphorus and decreased salinity in the lagoon, and probably to combined top-down and bottom-up processes in the marine gulf.

Plankton dynamics across the freshwater, transitional and marine research sites of the LTER-Italy Network: patterns, fluctuations, drivers

A first synoptic and trans-domain overview of plankton dynamics was conducted across the aquatic sites belonging to the Italian Long-Term Ecological Research Network (LTER-Italy). Based on published studies, checked and complemented with unpublished information, we investigated phytoplankton and zooplankton annual dynamics and long-term changes across domains: from the large subalpine lakes to mountain lakes and artificial lakes, from lagoons to marine coastal ecosystems. This study permitted identifying common and unique environmental drivers and ecological functional processes controlling seasonal and long-term temporal course. The most relevant patterns of plankton seasonal succession were revealed, showing that the driving factors were nutrient availability, stratification regime, and freshwater inflow. Phytoplankton and mesozooplankton displayed a wide interannual variability at most sites. Unidirectional or linear long-term trends were rarely detected but all sites were impacted across the years by at least one, but in many case several major stressor(s): nutrient inputs, meteo-climatic variability at the local and regional scale, and direct human activities at specific sites. Different climatic and anthropic forcings frequently co-occurred, whereby the responses of plankton communities were the result of this environmental complexity. Overall, the LTER investigations are providing an unparalleled framework of knowledge to evaluate changes in the aquatic pelagic systems and management options.

Multiannual Chattonella subsalsa Biecheler (Raphidophyceae) blooms in a Mediterranean lagoon (Santa Giusta Lagoon, Sardinia Island, Italy)

Recurrent blooms of Chattonella subsalsa (Raphidophyceae) were associated with fish kills in Santa Giusta Lagoon (Mediterranean Sea). This study investigated the population dynamics of C. subsalsa and its relationship with environmental and meteorological conditions, using multiannual ecological data (1990-2016). In addition, for the first time, this study examined the presence of C. subsalsa cysts in lagoon sediments. The species was first detected in Santa Giusta Lagoon in July 1994. Bloom events coinciding with fish kills were recorded in 1994, 1998, 1999, and 2010. The timing and dynamics of C. subsalsa blooms and fish kills varied over the examined period. Presence of C. subsalsa was strongly influenced by temperature, especially in the early years of the series (1990-2002). Temperature control may have been lesser important in the more recent years, when higher temperature may have generated continuative suitable conditions for C. subsalsa affirmation, especially in July. Thus, the variations in the availability of food (via autotrophy and/or mixotrophy) could be one of the control keys on the proliferation of this species in the future in SG. Cysts of C. subsalsa were present in lagoon sediments at abundances ranging 200-2000cystsg-1 wet sediment. This study is among a few that have examined C. subsalsa population dynamics and bloom events in the field over a long time period. Findings from this study contribute to a better understanding of C. subsalsa bloom development, by identifying environmental and meteorological variables that may promote blooms of this species in the Santa Giusta Lagoon.

Dataset of seasonal mean volumes of phytoplankton cell size classes in Mediterranean shallow coastal lagoons

In this article, the floristic lists and the seasonal mean cell volumes of phytoplankton taxa observed in three Mediterranean lagoons are reported. These datasets include 40 species, 67 other taxa identified at least at genus level, and further 13 taxa attributed only at order or class level. These data are associated with Pulina et al. “Seasonal variations of phytoplankton size structure in relation to environmental variables in three Mediterranean shallow coastal lagoons” (Pulina et al., 2018) [1], where phytoplankton taxa were included in two different cell size classes (Utermöhl fraction of phytoplankton, cell size > 3 µm; Picophytoplankton, cell size < 3 µm) and in which their seasonal variations were interpreted and discussed.