Layout 1 REVIEW Context and aims The Venice Lagoon (VL) (Figure 1) has an intrinsic eco- logical complexity due to its large size, marked habitat hetero- geneity and hydrodynamics. It is one of the most important Transitional Water Ecosystems (TWE) (European Union, 2000; Elliot and McLusky, 2002) in the Mediterranean. Mi- crotidal and polyhaline, it is located in the north-western sector of the Adriatic Sea. Covering an area of approximately 550 km², about 85% of the lagoon has depths of less than two me- ters, while about 10% consists of channels deeper than five meters. The VL is connected to the sea through three port inlets (Lido, Malamocco, and Chioggia) where water exchange is governed by the Adriatic tides, with amplitude ranges between 20 and 100 cm, during neap and spring tides respectively. Freshwater inflow into the lagoon originates from twelve main tributaries, with seasonal peaks occurring in spring and au- tumn. The VL is heavily influenced by urbanization, intensive industrial and port activities, and significant maritime traffic. The spatial and temporal dynamics of phytoplankton in TWE is affected and modulated by a wide range of local and regional factors (Cloern and Jassby, 2008; Winder and Cloern, 2010). Notable TWE’s features, such as shallow depths, close benthic–pelagic coupling and connectivity to both land and sea, significantly impact on the composition and distribution of phytoplankton across spatial and temporal scales (Cloern and Jassby, 2010). Understanding phytoplankton community structure and its fluctuations is essential for environmental management, as phytoplankton is a key water quality indicator in TWE under the European Water Framework Directive (WFD) (WFD 2000/60/ EC). Research on phytoplankton in the VL has been conducted since the mid-20th century with relevant but sporadic studies, covering different areas with irregular sampling frequency and limited time span, driven by heterogeneous targets and pur- poses. Systematic monthly surveys of hydrochemical parame- ters and phytoplankton were initiated only in 1998 at five stations located in the northern and central areas of the VL (Figure 1) (Bianchi et al., 2003). These stations were strategi- cally selected to represent distinct environmental conditions Literature-based insights into phytoplankton ecology at the Long-Term Ecological Research - Italy site Venice Lagoon Alessandra Pugnetti, Fabrizio Bernardi Aubry, Francesco Acri, Stefania Finotto CNR ISMAR, Venezia, Italy ABSTRACT Since 1998, phytoplankton communities have been systematically studied at five stations in the northern and central parts of the Venice Lagoon (VL). The long-term maintenance of this phytoplankton data series, alongside similar datasets for other communities (e.g., zoo- plankton, macrozoobenthos, and macrophytes), has contributed to the inclusion of the VL as a site in the Italian Long-Term Ecological Research (LTER) network. In this paper, we synthesize key insights into the phytoplankton ecology of the VL’s LTER stations, based exclusively on published studies. Specifically, we provide an overview of phytoplankton taxonomic composition, morphological traits, seasonal succession, long-term trends and interactions with the other components of the planktonic communities. Corresponding author: Fabrizio Bernardi Aubry, CNR ISMAR, Ar- senale Tesa 104, Castello 2737/F, 30122 Venezia, Italy. E-mail: fabrizio.bernardi@ismar.cnr.it Key words: phytoplankton, LTER-Italy, Lagoon of Venice, litera- ture review. Authors’ contributions: all the authors made a substantive intellec- tual contribution. All the authors have read and approved the final version of the manuscript and agreed to be held accountable for all aspects of the work. Conflict of interest: the authors declare no potential conflict of in- terest. Funding: none. Availability of data and materials: all data generated or analyzed during this study are included in this published article. Acknowledgments: this work was presented at the XXVII AIOL Congress (Napoli, June 26-30, 2023). The Venice Lagoon is in- cluded in the Italian (LTER- Italy), European (LTER-Europe) and International (LTER- International) Long-Term Ecological Re- search (LTER) networks: the phytoplankton time series were gath- ered in the context of these networks. The authors wish to thank L. Dametto and the crew of the M/B “Litus”, M. Penzo, D. Penzo and G. Zennaro for their technical support during samplings and fieldwork, along the years. Received: 21 March 2025. Accepted: 4 August 2025. Publisher’s note: all claims expressed in this article are solely those of the authors and do not necessarily represent those of their affil- iated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher. ©Copyright: the Author(s), 2025 Licensee PAGEPress, Italy Advances in Oceanography and Limnology, 2025; 16:13837 DOI: 10.4081/aiol.2025.13837 This work is licensed under a Creative Commons Attribution- NonCommercial 4.0 International License (CC BY-NC 4.0). A. Pugnetti et al.28 and anthropogenic impacts, influenced by the interaction be- tween marine and fresh inputs, as well as by urban, industrial and agricultural pollution. Like other TWEs, the primary producer’s community in the VL is highly diversified, comprising several functional lev- els, including phytoplankton, benthic microalgae, aquatic an- giosperms and macroalgae. However, in the selected areas as well as in other extensive regions of the VL, phytoplankton is the prevalent primary producer and play a crucial role in the lagoon’s ecosystem functioning (Acri et al. 2004; Sfriso et al. 2005; Bernardi Aubry et al., 2013). This paper aims to summarize key insights into the phyto- plankton ecology of the VL based on published literature from 2003 - the year of the first publication on the five stations - to 2024, derived from regular observations at these LTER sta- tions. Specifically, we provide an overview of phytoplankton taxonomic composition, morphological traits, seasonal succes- sion, long-term trends, as well as its interactions with other planktonic communities. Overview of the phytoplankton in the Venice Lagoon: taxonomic composition, seasonal succession, and morphological traits For general context, the chlorophyll a (chl), nutrient con- centration, and phytoplankton abundances are reported in Table 1. Among the various seasonal phytoplankton cycles ob- served in TWE, the annual pattern of phytoplankton abundance in the VL exhibits features typical of temperate, enclosed coastal ecosystems with shallow depths and permanently high nutrient concentrations (Cebrian and Valiela 1999). Specifi- cally, phytoplankton abundance follows a unimodal pattern, peaking in summer (July and August) (Bernardi Aubry et al, 2013, 2021, 2022). In the VL nutrient availability does not ap- pear to be a limiting factor throughout the year; instead, the seasonal variations of temperature and light are the main driver of the yearly phytoplankton growth. Figure 1. The Venice Lagoon and the location of the five LTER sampling stations (Bernardi Aubry et al., 2021). Phytoplankton ecology at the Venice Lagoon 29 The main ecological characteristics of the VL - turbulence, nutrient enrichment, and connectivity with surrounding systems such as land, sea, and sediments - play a more significant role in shaping phytoplankton communities than the specific attributes of each individual station (Bernardi Aubry et al., 2013). Conse- quently, the most abundant phytoplankton taxa are common across all the five LTER stations, regardless of their unique envi- ronmental conditions. The key phytoplankton taxa are listed in Table 2 (Bernardi Aubry et al., 2021). Diatoms dominate the VL phytoplankton community year-round and across all stations (Bernardi Aubry et al, 2013, 2021, 2022) and are the group that contribute to most phytoplankton blooms in the lagoon, which is consistent with the characteristics of permanently nutrient-rich and turbulent environments (Margalef 1978). Early winter is pri- Table 1. Averages (Avg) and standard deviations (SD), minimum (min) and maximum (max) values, and number of observations (n) for the main abiotic parameters in the 20-year period 1998-2017 (Bernardi Aubry et al., 2021). Avg Min Max SD n Transparency m 1.4 0.1 6.6 1.0 1169 Temperature °C 18.2 0.6 33.9 7.5 1220 Salinity 28.62 8.63 37.05 4.79 1218 Relative Oxygen % 105 38 204 18 1217 N-NH3 µM 8.74 0.01 69.23 9.30 1215 N-NO2 µM 1.49 0.02 11.93 1.11 1215 N-NO3 µM 32.49 0.01 211.14 28.51 1215 DIN µM 42.71 0.52 242.97 33.74 1215 Si-SiO4 µM 33.20 0.83 294.15 30.99 1215 P-PO4 µM 0.79 0.01 17.19 0.96 1215 N/P 118 1 1889 161 1215 Chlorophyll-a µg L−1 5.23 0.02 124.64 10.86 1214 Total Phytoplankton Cells L−1 4,471,079 92,293 115,496,858 9,706,009 1202 Table 2. List of main phytoplankton taxa in VL for each season in the period 1998–2017. For each taxon, we also indicate the divisions to which they belong. Taxa are listed in order of decreasing importance of abundance and frequency in each season (re-arranged from Bernardi Aubry et al., 2021). Division Species WINTER Diatoms Skeletonema marinoi Coccolithophorids Emiliania huxleyi Diatoms Gyrosigma fasciola Diatoms Navicula tripunctata Diatoms Pseudo-nitzschia spp. Diatoms Melosira nummuloides Diatoms Melosira spp. Diatoms Amphora lineolata Euglenophyceans Eutreptia globulifera Chrysophyceans Dinobryon coalescens SPRING Diatoms Navicula spp. Diatoms Navicula cryptocephala Dinoflagellates Prorocentrum minimum Diatoms Cyclotella spp. (small) Dinoflagellates Prorocentrum gracile Chlorophyceans Ankistrodesmus spp. Dinoflagellates Prorocentrum triestinum Diatoms Halamphora hyalina Diatoms Licmophora gracilis SUMMER Diatoms Cylindrotheca closterium Diatoms Thalassiosira spp. (small) Diatoms Nitzschia frustulum Dinoflagellates Und. naked dinoflagellates <20 µm Diatoms Thalassiosira pseudonana Dinoflagellates Gymnodinium spp. Division Species Diatoms Chaetoceros spp. (small) Diatoms Phaeodactylum tricornutum Diatoms Leptocylindrus minimus Diatoms Skeletonema tropicum Chlorodendrophyceans Tetraselmis spp. Dinoflagellates Blixaea quinquecornis Pyramimonadophyceans Pyramimonas spp. Diatoms Cyclotella caspia Diatoms Pseudo-nitzschia galaxiae Diatoms Cerataulina pelagica Diatoms Proboscia alata Diatoms Pseudo-nitzschia multistriata Dinoflagellates Protoperidinium bipes Diatoms Chaetoceros calcitrans Diatoms Chaetoceros compressus Chlorophyceans Scenedesmus quadricauda Diatoms Thalassiosira rotula Dinoflagellates Prorocentrum rhathymum Trebouxiophyceans Chodatella spp. Coccolithophorids Michaelsarsia adriatica Diatoms Chaetoceros diversus AUTUMN Diatoms Cocconeis spp. Diatoms Craticula cuspidata Coccolithophorids Calciosolenia murrayi Diatoms Placoneis elginensis Diatoms Sellaphora pupula Diatoms Coscinodiscus spp. A. Pugnetti et al.30 marily characterized by resuspended species, such as Gyrosigma fasciola and Melosira spp. In late winter, Skeletonema marinoi is dominant, followed in spring by resuspended diatoms such as Navicula spp., Halamphora hyalina, Licmophora gracilis, and Cyclotella spp. The summer diatom peaks are driven by a diverse assemblage, comprising Cylindrotheca closterium, Thalassiosira pseudonana, Nitzschia frustulum, Phaeodactylum tricornutum, various Chaetoceros spp., Pseudo-nitzschia spp., and Leptocylin- drus minimus. In autumn, benthic diatoms such as Placoneis el- ginensis, Navicula spp., and Cocconeis spp. characterize the season’s low abundance. Due to the strong benthic–pelagic cou- pling, benthic resuspension plays a fundamental role in VL diatom dynamics. Indeed, many of the taxa identified in the lagoon waters belong to the microphytobenthos and are resuspended from sed- iments. Notably, some diatom species responsible for summer blooms, such as Nitzschia frustulum and Thalassiosira pseudo- nana, are frequently found in benthic habitats and appear to thrive in both sediments and the water column (Facca et al., 2002; Facca and Sfriso, 2007). Other key diatom taxa, such as Skeletonema marinoi, Thalassiosira spp., Chaetoceros spp., are euplanktonic and also found in the adjacent Adriatic coastal waters. Actually, the VL and the Gulf of Venice, another LTER Italy site, form a complex, interconnected system. A comparative analysis of the two environments (Bernardi Aubry et al., 2022) revealed that they share several generalist taxa, although local environmental factors play a predominant role in structuring their respective phytoplank- ton communities. The second most abundant phytoplankton group in the VL consists of taxonomically diverse undetermined flagellates (nanoflagellates) smaller than 10 μm. These organisms are a constantly present and significant component throughout the year, with peaks in spring and summer. A similar feature has been observed in other transitional ecosystems (Glibert et al., 2010; Bec et al., 2011; Durante et al., 2013; Pachés et al., 2014; Leruste et al., 2016) and in the Adriatic Sea (Bernardi Aubry et al., 2012; Cabrini et al., 2012; Mozetic et al., 2012; Neri et al., 2022). The taxonomic identification of nanoflagel- lates using light microscopy remains a major limitation in rou- tine long-term phytoplankton monitoring. Metabarcoding analyses of VL planktonic protist communities (Armeli Mini- cante et al., 2020) have revealed a greater diversity within this group than previously recognized, identifying taxa such as Pic- ochlorum and Micromonas for the first time. Dinoflagellates, representing less than 2% of the total abundance, are most prevalent in spring (Prorocentrum spp.) and summer (Gymnodinium spp., Blixaea quinquecornis, Pro- toperidinium bipes, and Prorocentrum rhathymum). Coccol- ithophorids accounted only for a 0.2% average contribution to total abundance, with Emiliania huxleyi dominating, especially in September and winter, while summer and autumn samples are also characterized by the presence of Michaelsarsia adri- atica and Calciosolenia murrayi, respectively. Some freshwa- ter taxa are recorded sporadically and in very low abundance. In winter, these included the euglenophycean Eutreptia glob- ulifera and the chrysophycean Dinobryon coalescens; in spring, Ankistrodesmus spp. (chlorophyceans); in summer Tetraselmis spp. (chlorodendrophyceans), Pyramimonas spp. (pyramimonadophyceans), Scenedesmus quadricauda (chloro- phyceans), and Chodatella spp. (trebouxiophyceans). Despite the taxonomic diversity of the VL phytoplankton, a limited number of dominant morphological types can be rec- ognized (Bernardi Aubry et al., 2017). Seven morphological shapes are consistently widespread across space and time, fol- lowing two main adaptive strategies: (i) a high surface area- to-volume ratio (S/V) combined with a low Greatest Axial Linear Dimension (GALD), and (ii) a low S/V together with high GALD. Certain morphological traits, particularly those contributing to high S/V (ranging from 0.8 to >1), remain rel- atively constant: (i) small size (GALD < 15 μm), especially in round-shaped organisms, and (ii) elongated forms with high GALD, observed both within individual species and across dif- ferent taxa. This morphological structure aligns with the char- acteristics described by Reynolds (1994 and 2003) for kinetic lakes, where species assemblages are strongly influenced by frequent mixing. Two primary and coexisting ecological strate- gies indeed emerge: Type I (r or C)—small, unicellular nanoplanktonic algae capable of rapid, opportunistic growth when nutrients are available and waters are well-mixed, and Type II (w or R)—mostly non-motile species with elongated or irregular shapes that enhance light and nutrient acquisition but require water turbulence for suspension. Long-term trends in the VL water quality based on phytoplankton Published analyses (Acri et al., 2020; Bernardi Aubry et al., 2020) of long-term phytoplankton changes at the five LTER stations currently cover the first 20 years of monitoring (1998-2017). These studies indicate an overall improvement in the lagoon’s water quality, particularly a reduction in the trophic state. This trend is primarily attributed to decreased in- puts of dissolved inorganic nitrogen from the watershed, fol- lowing the implementation of the Council Directive 91/676/EC, which introduced restrictions on the application of nitrogen-rich farm waste. Additionally, increased water trans- parency and higher dissolved oxygen levels have been ob- served, largely due to the resurgence of macroalgae and seagrasses, whose biomass has grown following regulations on clam fishing. The general water quality improvement, however, was not directly reflected in phytoplankton abundance, which showed a declining trend that was not statistically significant (Bernardi Aubry et al., 2021). Instead, it was more effectively captured using the Multiparametric Phytoplankton Index (MPI), which integrates multiple indicators: Menhinick’s Diversity Index (measuring species richness; Menhinick, 1964), Hulburt’s Dominance Index (assessing community evenness; Hulburt et al., 1963), bloom frequency, and chl concentrations. Devel- oped specifically for the VL to assess water quality (Facca et al., 2014), the MPI has demonstrated its reliability, leading to its official adoption by the Italian Ministry of Ecological Tran- sition (formerly the Ministry of the Environment) to fulfil the requirements of the Water Framework Directive (WFD 2000/60/EC). Over the 20-year monitoring period, MPI trends indicate a shift in water quality from "moderate" to "good" conditions: during the first 12 years, moderate water quality was predominant, but from 2014 to 2017 the frequency of good water quality levels increased. A significant decrease in bloom frequency was recorded over the 20-year study, despite no Phytoplankton ecology at the Venice Lagoon 31 major changes in the composition of the phytoplankton com- munity. The dominant taxa within different groups remained largely consistent over the years (Bernardi Aubry et al., 2021). In particular, diatom blooms, regardless of intensity, were con- sistently composed of the same few species: Skeletonema marinoi in late winter, Thalassiosira spp. and Cyclotella spp. in spring, and Cylindrotheca closterium, Chaetoceros spp., and Nitzschia frustulum in summer. Phytoplankton and the trophic plankton web Studies conducted in the early 2000s, based on a few sea- sonal samplings, explored the relationships between phyto- plankton and other components of the planktonic trophic web. Regarding planktonic biomass (Coppola et al., 2006, 2007), autotrophic communities were dominated by pico- and nanoplanktonic fractions, while heterotrophic communities were primarily composed of picoplankton (bacteria). The over- all ratio of autotrophic to heterotrophic planktonic biomass in- dicated a predominance of heterotrophic biomass. By analysing the spatial and temporal co-occurrence of phyto- plankton, heterotrophic protists, and metazooplankton in rela- tion to biogeochemical properties, Bandelj and colleagues (2008) identified five multitrophic plankton assemblages in the VL. Seasonality played a significant role in structuring these communities: autotrophy dominated from late winter to spring in the inner lagoon, while heterotrophy prevailed from late summer to early winter, especially near the inlets. The strength of the coupling between phytoplankton and bacterial communities was also investigated (Pugnetti et al., 2010) by analysing Dissolved Primary Production (DPP) vs Bacterial Carbon Demand (BCD), Bacterial Carbon Production (BCP) vs Phytoplankton Production (PP), and Community Respiration (CR) vs PP. Results suggested that phytoplankton and bacterial communities were largely uncoupled, with BCD exceeding PP, and BCP often surpassing PP. This indicates that bacterial metabolism relies on additional organic carbon sources beyond phytoplankton production. During periods when CR exceeded PP, the planktonic system is primarily sus- tained by stored or imported organic matter. A recent study by Loschi and colleagues (2023), although focused only on sum- mer assemblages, further highlights detrital resuspension as a crucial factor in maintaining the lower trophic levels in the VL. Sediment resuspension allowed bacteria to sustain high densi- ties even when phytoplankton-derived carbon was insufficient. Bacteria thriving on detritus significantly influenced the food web by serving as a major food source for protozoa, which were then consumed by higher trophic levels. Conclusions “without a conclusion” This literature review has provided an overview of the cur- rent knowledge on phytoplankton communities in the VL, based on data collected from the five LTER stations, which have been sampled monthly since 1998. These sustained ob- servational efforts have provided a valuable temporal frame- work to detect ecological trends and assess phytoplankton dynamics in a highly variable coastal system. Looking ahead, maintaining these long-term studies, while strategically ex- panding research into key areas, will be crucial to deepen our understanding of the phytoplankton ecology and the whole la- goon’s ecosystem functioning. Future investigations in the VL should be informed by three intersecting dimensions: (i) the broader European con- text, where the European Long-Term Ecosystem Research In- frastructure (eLTER-RI) is being developed (Mirtl et al., 2021; Mollenhauer et al., 2018; Ohnemus et al., 2025; Zacharias et al., 2025), offering a comprehensive conceptual and methodological framework; (ii) the main on-going re- search activities and priorities emerging from other LTER- Italy aquatic sites (Capotondi et al., 2021), which enable cross-site comparisons and shared analytical frameworks; and (iii) the local context, including site-specific scientific ques- tions, existing research efforts, and infrastructural capacity within the lagoon. At the intersection of these levels, we identified four key research directions, aligning with core themes of eLTER-RI, reflecting broader ecological challenges and incorporating ac- tivities and perspectives of other LTER-Italy sites. Carbon cycling and allocation Understanding the role of phytoplankton in the carbon cycle requires a detailed examination of their interactions with other trophic compartments across lagoon zones that differ in hydrodynamics and nutrient regimes. Investigating carbon fluxes will enhance our capacity to model ecosystem metabo- lism and assess the lagoon’s role in coastal carbon budgets. Molecular approaches to biodiversity The application of high-throughput molecular techniques, such as DNA metabarcoding and imaging flow cytometry, will provide finer taxonomic resolution and improved insight into the structure and dynamics of protistan communities. Particular attention should be directed toward nanoflagellates, a key yet understudied component of the lagoon plankton, whose diver- sity and ecological roles remain largely unknown. Modelling of environmental and ecosystem change Strengthen the link between existing and newly developed hydrodynamic models and phytoplankton distribution patterns will improve the understanding of spatial and temporal vari- ability, allowing the analysis to be extended to broader areas of the lagoon. Remote sensing for phenological monitoring The use of remote sensing technologies offers novel op- portunities to track spatial and temporal patterns in phyto- plankton dynamics and environmental drivers. These approaches can complement in situ observations and enable high-resolution, large-scale monitoring of ecosystem changes over time. Together, these research lines provide a roadmap for ad- vancing phytoplankton ecology in the VL and strengthening its integration within the national and European LTER frame- A. Pugnetti et al.32 works. 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