**Preface to "Zooplankton Diversity and Pelagic Food Webs: Investigating Present and Past with Different Techniques"**

Zooplankton is of key importance in the structure and functioning of aquatic food webs, providing a large part of the functional and structural biodiversity of predator and prey plankton communities. Promptly responding to long-term and seasonal changes in the physical and chemical environment, zooplankton organisms are sensitive indicators of patterns and mechanisms of impact drivers, both natural and human induced. In this volume, we aim to present evidence for both long-term and seasonal changes in zooplankton community structure and dynamics, investigating different approaches from population dynamics to advanced molecular techniques and reconstructing past communities from subfossil remains in lake sediments.

By applying an innovative statistical technique, Arfe et al. were able to disentangle the long-term ` impact of changes in trophic status, nutrients supply and climate in a large, deep subalpine lake. Patterns of change in zooplankton and phytoplankton communities at a taxonomic and functional level were addressed, and the role of the two main drivers (climate and trophy) was identified along with different phases of the lake's long-term evolution.

Fundamental ecosystem alteration in high-latitude lakes is of basic importance for holistic understanding of lake responses and resilience to climate warming. In their contribution to this volume, Nevalainen et al. investigated long-term changes in the functioning of a Tundra Lake (Lake Loazˇzej ˇ avri) by means of a paleolimnological approach, based on Cladocera subfossil remain ´ analyses. The study reconstructed taxonomic and functional changes through a sediment core representing 2000-year. Variations in functional diversity were associated with climate oscillations. Higher diversity during warm climate periods suggested the establishment of physical UV refugia for cladocerans by the benthic vegetative substrata under increasing benthic production. A decreasing trend of *Bosmina* sexual reproduction with increasing benthic production also traced a favorable environmental regime since the 17th century.

Changes in planktivory and herbivory regimes over the last 250 years were the object of a paleolimnological study by Carrozzo et al., focused on a shallow lake in Argentina (Lake Blanca Chica). The authors applied Generalized Additive Models (GAM) to time series of fish predation indicators (ephippial abundance and size, micro size, fish scales, and the planktivory index) and pheophorbide α concentration. Profound changes in taxonomic composition and in the size structure of the zooplankton community, and of grazers in particular, resulted in changes in planktivory regime during lake eutrophication. The results of this study provide a reference status for future management strategies of this ecosystem.

High mountain lakes are biodiversity treasures and host endemic taxa that are adapted to live in extreme environments. Among adaptations, production of diapausing eggs allows for harsh conditions to be overcome during the cold season. These sedimentary resting eggs can provide a reservoir of species, thus buffering from extinction risk and biodiversity loss. In the contribution by Piscia et al., the viability of diapausing eggs of Rotifers and Cladocera from sediment cores, representing the last ca. 1100 and 500 years of two lakes in the Khumbu Region of the Himalayas, was tested in the laboratory setting. It was found that only diapausing eggs of the Monogont rotifer *Hexarthra bulgarica nepalensis* were able to hatch, thus suggesting that a permanent egg bank is lacking for the other taxa of the lakes, not least for the two *Daphnia* species described from these sites. The different ability of different taxa to leave viable diapausing eggs in the sediments of high mountain lakes poses serious constraints to the capability of buffering risk of biodiversity loss in these extremely fragile environments.

Body size is a major trait for zooplankton vulnerability to fish predation. The study by Czerniawski and Krepski compared abundance and composition of the zooplankton community in outlet sections of a lake and a waste stabilization pond of a sewage treatment plant. The aim of the study was to determine which zooplankton organisms, among those drifting from the waste stabilization pond or from the lake, were preferentially preyed by roach. Large bodied *Daphnia pulex* was the most abundant in the pond, while small planktonic rotifers prevailed in the lake. The results indicated that fish preferentially fed on zooplankton drifting from the waste stabilization pond than from the lake. The importance of riverine zooplankton in the downstream food web may render these results even more important.

Understanding the impact of environmental drivers on the regional scale on biodiversity is crucial for planning effective biodiversity conservation strategies. Mancinelli et al. used a proxy of phylogenetic diversity of crustacean zooplankton assemblages from 40 ponds and small lakes located in Albania and North Macedonia to assess the level of prediction of changes from waterbodies' landscape characteristics and local bioclimatic conditions. The species–area relationship hypothesis proved inadequate in explaining the diversity of crustacean zooplankton assemblages; in contrast, local biological and climatic factors appeared to influence species richness and composition, but not phylogenetic diversity. The latter is in fact a result of long-term adaptation mechanisms and, as such, is hardly predictable and largely unreplaceable.

Coastal lagoons are very important ecosystems, still poorly studied, particularly with respect to the impact of changes in water quality. In the paper by Oghenekaro and Chigbu, mesozooplankton abundance, diversity and community structure were analyzed in time and space with respect to changes in salinity and temperature. Diversity was higher at sites with high salinity near the Ocean City inlet than at sites near the mouth of tributaries with lower salinity, higher nutrient levels and higher phytoplankton biomass. It was hypothesized that the relatively low salinity and high temperatures in 2013 were responsible for the increase in abundance of hydromedusae, in turn resulting in decreased abundance of bivalve larvae and other taxa, because of increased predation.

Climate change is expected to influence population dynamics of copepods, and effects are likely more pronounced for marine species at high latitudes. The study by Balazy et al. focuses on key zooplankton species of the genus Calanus in the European Arctic. Analyzing changes in body size and phenology of populations from two fjords on the West Spitsbergen Shelf (cold Hornsund vs. warm Kongsfjorden), they found that in Hornsund, the share of *Calanus* in the zooplankton community was greater, and the copepodite structure was progressively older over time, matching the little auks (*Alle alle*; seabird) timing.. The importance of *Calanus* was much lower in Kongsfjorden, and with a prevalence of young copepodites; this result was explained by the Atlantic water advections, which made this area a less favorable feeding ground. Altered age structure towards a domination of young copepodites and the body size reduction of *Calanus* with increasing seawater temperatures was interpreted as resulting in insufficient food availability for these seabirds, a reason for recommending further studies on match/mismatch between *Calanus* and little auks.

Exotic species invasion is recognized as a potential hazard for biodiversity conservation and ecosystem services. Camatti et al. describe the history of the invasion and the present distribution of the copepod *Acartia tonsa* in the Venice lagoon. Temporal trends and changes in coexistence patterns among original *Acartia* species before and after *A. tonsa* settlement and dominance are discussed in their contribution. Disappearance of local species previously dominant and a possible coexistence with the invader are analyzed in detail, highlighting the environmental factors promoting the success of the invasive species. An analysis of spatial distribution of *A. tonsa* revealed significant association with temperature, phytoplankton, particulate organic carbon (POC), chlorophyll α, and counter gradient of salinity. The study overall confirms *A. tonsa* as an opportunistic tolerant species.

Protists, basic components of plankton communities, are the object of the study by Chen et al. Aiming at investigating the impact of sand dust and phosphorus (P) on the microbial food web of the southern Yellow Sea, laboratory experiments investigated changes in the protists community exposed to different concentrations of sand dust and P. The study showed that the effects of the addition of sand dust and phosphorus on size fractions and on autotrophic vs/ mixotrophic protists were similar and dependent on exposure time. Phosphorus addition initially promoted growth of autotrophs, while at the late period of sand dust deposition, small-size heterotrophic protists contributed to the material cycle and food transmission in the ocean. The positive effect of sand dust deposition on autotrophic plankton of the southern Yellow Sea, naturally P deficient, was attributed to a release of P from sand dust. In turn, the release resulted in alteration of the structure of the micro-food web, therefore affecting the ecological function of planktonic protists in the system.

Zooplankton is crucial for the transfer of matter, energy and pollutants through aquatic food webs. Such transfer varies with the season, along with changes in contribution of primary and secondary consumers to zooplankton abundance and standing stock biomass. In the paper by Piscia et al., taxa- and size-specific carbon and nitrogen stable isotope analysis is applied for tracing the path of pollutants through zooplankton along the seasons. Plurennial changes in concentration of polychlorinated biphenyls (PCBs) and dichlorodiphenyltrichloroethane and its relatives (DDTs) were analyzed along with size- and taxa-specific δ<sup>15</sup>N signatures in a large deep, subalpine lake. The highly significant correlation between taxa-specific δ<sup>15</sup>N and pollutant concentrations, resulting from taxa contribution to standing stock biomass and δ<sup>15</sup>N isotopic signatures, proved a basic step in understanding the taxa-specific role in zooplankton enrichment in PCBs and DDTs and the seasonal dynamics of pollutants' transfer to planktivorous fish.

Mining is recognized to deeply influence invertebrate assemblages in aquatic ecosystems. The study by Pociecha et al. used Cladocera fossils in sediment cores to reveal the effects of cessation of mine water discharge on a Cladocera community structure and density. The aquatic system investigated includes the river and associated subsidence ponds in the valley. Some ponds were contaminated during the period of mining, which ceased in 2009, while one of the ponds only appeared after mining had stopped. The concentrations of Zn, Cd, Cu and Pb were much higher in sediments of the ponds formed during peak mining than in the ponds formed after the closure of the mine. Statistical analysis (CCA) showed that *Alonella nana, Alona affinis, Alona* sp. and *Pleuroxus* sp. were strongly correlated with pond age and did not tolerate high concentrations of heavy metals (Cu and Cd). Overall, the study provides evidence that the rate of water exchange by the river flow and the presence of aquatic plants affect species composition more than pond age itself.

The studies provide evidence of the importance of zooplankton in trophic webs of both fresh and salt waters. Providing new useful tools for better manage water ecosystems, these studies also highlight the need to improve our knowledge of zooplankton ecology and factors and mechanisms regulating their relationships with the environment in order to preserve the aquatic ecosystems that are increasingly impacted by human activities in the Anthropocene age we are living in.

> **Roberta Piscia, Roberta Bettinetti, Barbara Leoni, Marina Manca** *Editors*
