Dissertation committee:
Οικονόμου-Αμίλλη, Ομότιμη Καθηγήτρια, Τμήμα Βιολογίας, ΕΚΠΑ
Γιώργος Τσιρτσής, Καθηγητής, Τμήμα Ωκεανογραφίας και Θαλασσίων Βιοεπιστημών, Πανεπιστήμιο Αιγαίου
Διονύσιος Ε. Ραΐτσος, Επίκουρος Καθηγητής, Τμήμα Βιολογίας, ΕΚΠΑ
Περσεφόνη Μεγαλοφώνου, Καθηγήτρια, Τμήμα Βιολογίας, ΕΚΠΑ
Μαρία Μουστάκα, Καθηγήτρια, Τμήμα Βιολογίας, ΑΠΘ
Κωνσταντίνος Κορμάς, Καθηγητής, Σχόλη Γεωπονικών Επιστημών, Πανεπιστήμιο Θεσσαλίας
Σοφία Σπαθάρη, Λέκτορας, School of Life Sciences, University of Glasgow
Summary:
The purpose of this thesis is the ecological study of benthic diatom communities in coastal areas of the Saronicos Gulf. Particular emphasis was placed on the identification and taxonomic classification of benthic diatom species, which was necessary for the completion of the study. In addition, the composition of the community (biomass, diversity and abundance), seasonal succession and spatial distribution of benthic communities in the Saronicos Gulf were examined. A further objective of this thesis was to evaluate the potential of benthic sections for detecting eutrophication-induced coastal water quality differences in an oligotrophic system (such as the eastern Mediterranean). Finally, the thesis aimed to investigate and suggest diatom species and genera, which may serve as bio-indicators for assessing coastal water quality.
The choice of coastal areas of the Saronicos Gulf as a subject of study was very interesting. Coastal ecosystems are known to play an important ecological role, host a high number of species and greatly contribute to global productivity (Cloern et al., 2013). These ecosystems are also particularly important from a socio-economic point of view because they are associated with activities such as recreation, fishing, tourism, etc., and are also subject to severe anthropogenic pressures (urbanization, agriculture). It is precisely because of these intense activities that there is a risk of eutrophication, especially in particularly oligotrophic systems, such as the southeastern Mediterranean (Tsirtsis et al., 2008). In such ecosystems the species composition (primary producers - microflora) reflects the low nutrient concentrations that prevail throughout the year, while being sensitive to abrupt changes with increased nutrient loads, resulting in changes in the composition of the bio-community (Hillebrand & Kahlert 2001, Spatharis et al. 2007a). It was therefore considered very important to seek and establish a suitable biological means for monitoring the quality of coastal ecosystems (biomarkers).
Diatoms (Bacillariophyceae, diatoms) are the most numerous group of microbes, which due to their worldwide and widespread distribution in aquatic environments, have been established as indicators for assessing water quality, both in inland waters with the use of benthic diatoms and in lakes with phytoplankton species as wells as in coastal waters (phytoplankton species) - Water Framework Directive 2000/60 / EC (EC, 2000). It is also noted that phytoplankton are generally an efficient indicator for coastal eutrophication estimation (Spatharis & Tsirtsis, 2010). However, due to their passive movement from the marine currents, phytoplankton species cannot reflect and identify (hotpoints) sources of pollution and eutrophication. On the contrary, benthic diatoms, precisely because of their limited ability to move and their rapid response to changes in nutrient concentrations are potentially the appropriate organisms to detect at a lower spatial level the status of an ecosystem through changes in their abundance, composition and diversity (Morin et al., 2016). Very little information is available on benthic diatoms as biomarkers in coastal ecosystems and in particular oligotrophic coastal systems. Furthermore, despite the importance of marine benthic diatoms in the functioning of coastal ecosystems (MacIntyre et al. 1996; Cahoon 1999), little is known about the environmental factors that control the structure and composition of their communities (Cibic et al. 2007a, 2007b, Cibic & Blasutto 2011). Previous studies have focused primarily on tidal-controlled ecosystems (Agatz et al., 1999), estuaries, and brackish waters such as the Venice Lagoon (Facca & Sfriso 2007). In such systems, salinity and light play an important role in the structure of benthic diatom communities (Hillebrand & Sommer 1997, Weckström & Juggins 2005, Ulanova et al. 2009, Du et al. 2017). Primary producers in oligotrophic ecosystems appear to be less affected by the limitation of light and more by nutrient concentrations (Moore et al., 2013). In this context, the environmental factors that lead to the alteration of the benthic communities of diatoms, as well as the knowledge of the benthic species self-ecology along nutrient gradients, have been largely overlooked (Cibic & Blasutto 2011; Desrosiers et al. 2013). The contribution of knowledge in this direction is expected to help in the development of benthic bio-indicators, which will serve as an effective tool for assessing water quality in coastal ecosystems.
During this study, sampling was carried out between June 2006 and June 2007 at six (6) stations along the Saronicos Gulf, at approximately 2 weeks’ intervals, with a total of 26 excursions. An attempt was made to collect the samples on windless days in order to collect as much undisturbed substrate as possible. Samples were taken from the sub-coastal zone at a depth of 1-4 m. Plexiglas tubes with 3.3 cm diameter were used to collect the samples (sediment cores), and 1-liter plastic bottles were used to collect the water samples. Three (3) repeat samples (material and water) were taken from each station, while measurements were made in the field of parameters such as light radiation, temperature, salinity, pH conductivity, and dissolved oxygen. In the laboratory chlorophylls a and c, organic matter and chemicals were determined (according to the protocols described in Strickland & Parsons 1967, Standard Methods 1980, Parsons et al. 1984 for phosphorus, silicon, nitrate, nitrite and ammonia).
The taxonomic part consisted of 2 stages. The first stage involved identifying and classifying benthic diatom species, while the second involved quantitative determination (absolute abundance). The diatoms samples were then treated with concentrated acids using the classic protocols of Hasle and Fryxell (1970) and Schrader (1973 and three replicates slides of 250 μL were mounted using Naphrax© resin and 250 fields were counted in each slide. This quantitative method ensured the estimation of total abundance. Frustule counts were carried out using a Carl Zeiss photonic microscope at x1000 magnification and pictures were taken with a Canon G7 digital camera. Taxonomic identification to species level was carried out using reference floras (Hustedt, 1930, 1964, Hendey, 1964, Round et al., 1990, Witkowski et al., 2000, Lange-Bertalot 2001, Weckstrom & Juggins, 2006, Bukhtiyarova, 2006, Levkov, 2009, Louvrou et al., 2012). A total of 216 samples were analyzed and more than 24,000 frustules were counted. 91 genera and 448 diatom species were identified. Most taxa were rare, with only 284 taxa contributing over 1% to absolute abundance.
The study of abiotic parameters revealed that there is a direct correlation between rainfall and nutrient concentrations resulting in coastal waters, and that these are directly influenced by urban and agricultural runoff of adjacent areas. Furthermore, the results of the study showed that abundance and biomass of the community respond to nutrient changes caused by human activities (nitrogen, phosphorus) with a delay of 2-4 weeks.
The results of this study indicate that changes in dissolved inorganic nitrogen (DIN) concentrations caused by anthropogenic activities shape the composition of the benthic diatoms primarily spatially affecting their abundance, biomass and diversity. These characteristics of benthic diatom communities are sensitive (secondary) to temporal/ seasonal variations due to other environmental parameters (temperature, dissolved oxygen, pH, etc.) that affect the composition of the community but to a lesser extent. Nitrogen concentrations indicate a strong association with specific species and genera of diatoms, suggesting that these taxa could be bio-indicators of eutrophication, from anthropogenic activities, in coastal oligotrophic ecosystems. Specifically, 37 species of diatoms showed a positive correlation with nitrogen (6 species of Cocconeis and 4 species of Tryblionella) while 4 species showed a negative correlation with nitrogen of which 2 species of the genus Mastogloia. Species that showed a positive association with DIN were typical of station S3 (Skaramagas), which exhibited the highest DIN values throughout the year. Species that showed a negative correlation with DIN were typically at station 1 (Sounio), which is presented as the least congested (reference station). Specifically, species of the genus Mastogloia showed their highest concentrations during the summer and autumn months where DIN concentrations were lower. It seems that the study of benthic diatom communities could provide a more detailed spatial analysis of coastal eutrophication than traditional approaches such as phytoplankton biomass and diversity.
We recommend the use of total abundance as well as abundances of genera Cocconeis and Trybillonella as reliable indicators of eutrophication by enriching nutrients in oligotrophic systems. The genus Mastogloia can be used as an indicator of low nutrient concentrations in undisturbed areas. This study is the first to indicate the importance of these genera / species in the detection of local nutrient hot spots and pristine areas in the marine coastal ecosystem. Future research would also be interesting to focus further on the self-ecology of these species.
