Transitional waters (TWs), such as coastal lagoons, are bodies of surface water at the transition between saline and freshwater domains. These environments play a vital role in guaranteeing ecosystem services, including provision of food, protection against meteorological events, as anthropogenic carbon sinks, and in filtering of pollutants. Due to the escalating overpopulation characterising coastlines worldwide, transitional systems are over-exploited, degraded, and reduced in their macroscopic features. However, information on the impact of anthropogenic pressures on planktonic organisms in these systems is still scanty and fragmented. Herein, we summarise the literature, with a special focus on coastal lagoons undergoing anthropogenic pressure. Specifically, we report on the implications of human impacts on the ecological state of plankton, i.e., a fundamental ecological component of aquatic ecosystems. Literature information indicates that human forces may alter ecosystem structures and functions in coastal lagoons, as in other TWs such as estuaries, hampering the phytoplankton–zooplankton link, i.e., the main trophic process occurring in those communities, and which sustains aquatic productivity. Changes in the dominance and lifestyle of key planktonic players, plus the invasion of ‘alien’ species, and consequent regime shifts, are among the most common outcomes of human disturbance.

Plankton communities include both unicellular and multicellular organisms. An important unicellular component is represented by those protists (i.e., unicellular eukaryotes) that are non-strictly autotrophic organisms and consume bacteria and other protists. These organisms are an important link between primary producers and metazoans and are usually known as microzooplankton, protozooplankton, or mixoplankton, as many of them couple phagotrophic and photoautotrophic behaviours. Herein we report on the diversity of these organisms sampled at two sampling sites (coastal and offshore stations), at two depths (0 and 10 m), in the Gulf of Naples during the early autumn of 2020. Despite efforts to list plankton biodiversity of primary producers and metazoan grazers made in this area so far, protistan grazers are still poorly investigated and previous information date back to decades ago. Our survey identified dinoflagellates and oligotrich ciliates as the most abundant groups, while tintinnids were less quantitatively relevant. The taxonomic composition in samples investigated herein remarked that reported by previous studies, with the sole exception of the tintinnid Ascampbeliella armilla, which was never reported before. A coastal-offshore gradient in the taxonomical composition of protistan grazers was also observed, with some species more abundant within coastal waters and other better thriving in offshore ones. Surface and sub-surface communities also differed in terms of species composition, with the deeper communities in the two sites being more similar reciprocally than with communities at the surface. These differences were associated with distinct environmental conditions, such as light availability, as well with the standing feeding environment, arising potential implications in the functioning of the planktonic food web at the local scale.