Phytoplankton Blooms, Red Tides and Mucilaginous Aggregates in the Urban Thessaloniki Bay, Eastern Mediterranean

Abstract

We investigated the plankton community composition and abundance in the urban marine environment of Thessaloniki Bay. We collected water samples weekly from March 2017 to February 2018 at the coastal front of Thessaloniki city center and monthly samples from three other inshore sites along the urban front of the bay. During the study period, conspicuous and successive phytoplankton blooms, dominated by known mucilage-producing diatoms alternated with red tide events formed by the dinoflagellates Noctiluca scintillans and Spatulodinium pseudonoctiluca, and an extensive mucilage aggregate phenomenon, which appeared in late June 2017. At least 11 known harmful algae were identified throughout the study, with the increase in the abundance of the known harmful dinoflagellate Dinophysis cf. acuminata occurring in October and November 2017. Finally, a red tide caused by the photosynthetic ciliate Mesodinium rubrum on December 2017 was conspicuous throughout the sampling sites. The above-mentioned harmful blooms and red tides were linked to high nutrient concentrations and eutrophication. This paper provides an overview of eutrophication impacts on the response of the unicellular eukaryotic plankton organisms and their impact on water quality and ecosystem services.A large volume of domestic and industrial wastes from the city of Thessaloniki has been directed for decades to the Thermaikos Gulf and especially its inner part, Thessaloniki Bay. In the 20 th century, these wastes were discharged in the Bay without treatment, causing the eutrophication of the system. Since 2001, wastewater treatment has been implemented, decreasing the effects of anthropogenic eutrophication [17]. Although it is generally accepted in the public that the water quality in the Thermaikos Gulf has been improved compared to 20-30 years ago [18,19], the urban front of the Thessaloniki Bay, with restricted water circulation and shallowness, still exhibits apparent red tides and algal blooms. These events are usually the cause of irritation to the public, often mentioned in the Greek media, with subsequent socio-economic consequences to the city of Thessaloniki, especially the touristic center. Despite the growing concerns of the citizens and authorities on the water quality of the Bay and particularly of the urban front, only scarce and isolated studies have been published on the abundance and dynamics of plankton community (both phyto-and protozooplankton) in the urban part of the Gulf [20,21]. On the other hand, several studies in the broader Thermaikos Gulf have focused on phytoplankton [18,22,23] and the occurrence of HABs [24,25]. However, comprehensive studies on red tides and mucilage aggregate phenomena are lacking for the Thermaikos Gulf.According to the related legislation for the ecological water quality based on nutrient pressures and the phytoplankton quality element (Water Framework Directive; WFD, 2000/60/EC) there is a need for systematic and frequent monitoring of coastal waters. Furthermore, similar to the WFD objectives are those...

Patterns in Alpha and Beta Phytoplankton Diversity along a Conductivity Gradient in Coastal Mediterranean Lagoons

“…Blooms of the other euryhaline species, the mucilage producing diatoms Cylindrotheca closterium, Rhizosolenia setigera and Chaetoceros sp. have been observed in a eutrophic coastal sea contributing to frequent mucilage events [60].…”

Variability and Community Composition of Marine Unicellular Eukaryote Assemblages in a Eutrophic Mediterranean Urban Coastal Area with Marked Plankton Blooms and Red Tides

“…Here, we use HTS to investigate the marine unicellular eukaryotic diversity and spatial distribution throughout a year of frequent samplings in a coastal eutrophic Mediterranean urban area (Thessaloniki Bay). [32,33]. This approach was used as a follow-up to a previous publication examining via classical microscopy tools the microbial plankton communities in the same area and the same sampling period, which was characterized by marked harmful algal blooms, red tides, and mucilage aggregate incidents [33].…”

“…[32,33]. This approach was used as a follow-up to a previous publication examining via classical microscopy tools the microbial plankton communities in the same area and the same sampling period, which was characterized by marked harmful algal blooms, red tides, and mucilage aggregate incidents [33]. The Thessaloniki Bay is the inner part of Thermaikos Gulf-located in northern Greece-and has been accepting for decades a large volume of domestic and industrial wastes from the one million residents of the city of Thessaloniki.…”

“…The Thessaloniki Bay is the inner part of Thermaikos Gulf-located in northern Greece-and has been accepting for decades a large volume of domestic and industrial wastes from the one million residents of the city of Thessaloniki. Although wastewater treatment has been implemented, decreasing the effects of anthropogenic eutrophication [34], the restricted water circulation and shallowness of the Bay in combination with high nutrient inputs, lead to harmful algal blooms, apparent red tides, and mucilage aggregates [33], leading to growing concerns of the citizens and authorities on the water quality of the Bay and particularly of the urban front. Data on the abundance and dynamics of plankton community (both phyto-and protozooplankton) in the urban part of the Gulf obtained by classical microscopy [32,33,35], have attempted to identify and characterize the marine protist abettors and perpetrators responsible for these phenomena.…”

“…Although wastewater treatment has been implemented, decreasing the effects of anthropogenic eutrophication [34], the restricted water circulation and shallowness of the Bay in combination with high nutrient inputs, lead to harmful algal blooms, apparent red tides, and mucilage aggregates [33], leading to growing concerns of the citizens and authorities on the water quality of the Bay and particularly of the urban front. Data on the abundance and dynamics of plankton community (both phyto-and protozooplankton) in the urban part of the Gulf obtained by classical microscopy [32,33,35], have attempted to identify and characterize the marine protist abettors and perpetrators responsible for these phenomena. These studies have shown the contribution of taxa known to thrive due to eutrophication and to cause harmful events, such as the diatoms Cylindrotheca closterium, Skeletonema costatum, Chaetoceros spp., and the dinoflagellates Noctiluca scintillans and Dinophysis acuminata.…”