George Tsirtsis

Quantification of the effects of nonpoint nutrient sources to coastal marine eutrophication: applications to a semi-enclosed gulf in the Mediterranean Sea

An integrated modelling approach is proposed for the assessment of the nutrient loading of a coastal marine ecosystem from terrestrial sources. The evaluation of the model was based on a data set collected on a monthly basis from a Greek gulf surrounded by an intensively cultivated watershed. It consisted of three interacting components, that is the terrestrial, the hydrodynamic and the biological submodels. A modification of the loading-functions approach formed the basis of the terrestrial submodel, used for the estimation of the nutrient fluxes due to agricultural run-off. Nutrient loads from the atmosphere, the domestic effluents and the industrial activity were also considered. The hydrodynamic submodel, based on the Princeton Ocean Model, was used for the estimation of the spatial transport of nutrients and organic carbon. The third component was the biological submodel that was focused on the interactions of nitrogen, phytoplankton, zooplankton, bacteria and organic carbon. The model has shown good fit to the experimental data and it was further used for the assessment of the role of various exogenous and endogenous sources of nutrients and organic carbon. It was found that nutrient loading from the agricultural run-off was remarkable during winter, the contribution being between 40 and 60% of the total nitrogen stock. The fluxes of nitrogen and organic carbon from sewerage and industrial activity were also quantified and they were estimated to be up to 10% of the total stock whereas, the contribution of the wet and dry deposition from the atmosphere was insignificant. The integrated modelling approach could also be used for the understanding of the energy flow through the marine food web and the testing of various scenaria concerning the development of the coastal zone.

The effects of episodic rainfall events to the dynamics of coastal marine ecosystems: applications to a semi-enclosed gulf in the Meditteranean Sea

A modelling procedure of hourly time discretisation is developed in order to study the effects of episodic rainfall events to the dynamics of coastal marine environments. This methodology was applied on the gulf of Gera, Island of Lesvos, Greece, a semi-enclosed and shallow ecosystem surrounded by an intensively cultivated watershed. The simulation of biological and chemical processes in the water column was focused on the interactions of inorganic nutrients, primary producers and organic matter. The hydrodynamic behaviour of the area was estimated by the combination of the Princeton Ocean Model and a multiple-box mass balance approach in order to describe the tidal hydraulic regime. The estimation of nutrient fluxes due to non-point sources was based on a modification of the unit-mass response functions. The results have shown that nutrient loading from agricultural runoff could be considerable after episodic rainfall events, resulting in an increase of the nutrient stock from 40% to 60%. The responses of the primary producers are regulated from weather conditions of the following period and the residence time of excessive nutrient loads within the gulf that ranges from a time span of less than 10 days to 1 month period. The good fit between simulated and experimental data supports the view that this integrated modelling approach should be considered as a realistic reproduction of the dynamics of coastal marine ecosystems and be used for testing various scenarios concerning their sustainability.

Evaluation of Phytoplankton Community Indices for Detecting Eutrophic Trends in the Marine Environment

Twenty two diversity, evenness and dominance indices based on phytoplankton community structure were examined for their sensitivity to assess eutrophic conditions. Data from two sampling sites, one eutrophic and one oligotrophic, were used as reference data-sets for evaluating the indices. McNaughtons dominance index, cell number, species number (Hills N0), Kothes species deficit, Odums species per thousand individuals and Evenness E2, E3 seemed to be the most sensitive indices for discriminating between eutrophic and oligotrophic conditions. A case study was also performed using a data-set from ten stations spaced along the coastal area of the city of Rhodes, Aegean Sea, Greece to find out which of the indices were appropriate to assess different levels of eutrophication. Kothes species deficit, Hills N0 (species number), Margalefs and Gleasons indices have shown the highest sensitivity to resolve oligotrophic, mesotrophic and eutrophic conditions.

NUTRIENT SCALING FOR ASSESSING EUTROPHICATION: THE DEVELOPMENT OF A SIMULATED NORMAL DISTRIBUTION

A statistical methodology based on probabilistic procedures is proposed for assessing eutrophication. A simulated normal distribution was developed for nitrate concentrations, based on the information derived from two data sets. The first one is characteristic of oligotrophic conditions, and the second of eutrophic conditions. The distribution parameters were calculated from the raw data without distorting their information by applying transformations or any other drastic data manipulation. A standard normal distribution was derived and was evaluated in a case study. The proposed distribution can be used to define eutrophication scales, assign critical values, and test any nitrate concentration data for the evaluation of the trophic conditions of the area in which they were collected.

Distribution and ecology of Demospongiae from the circalittoral of the islands of the Aegean Sea (Eastern Mediterranean)

The distribution and ecology of the sponge fauna of the circalittoral zone of the Aegean archipelago were studied in the present work. Samples of 64 species of Demospongiae were collected with a sponge-fishing trawl on a network of 23 stations. The hard coralligenous bottoms of the Aegean seem to support the growing of a large number of sponge species with relatively high diversity, whereas rather low abundances were observed for the species collected on soft substrates. Spatial differences were observed in species number and abundance between Northern and Southern Aegean, possibly related to the geomorphological characteristics of the study area. The bathymetric distribution of sponge species in relation to the availability of light in the oligotrophic Aegean is also discussed.

Eutrophication risk assessment in coastal embayments using simple statistical models

A statistical methodology is proposed for assessing the risk of eutrophication in marine coastal embayments. The procedure followed was the development of regression models relating the levels of chlorophyll a (Chl) with the concentration of the limiting nutrient--usually nitrogen--and the renewal rate of the systems. The method was applied in the Gulf of Gera, Island of Lesvos, Aegean Sea and a surrogate for renewal rate was created using the Canberra metric as a measure of the resemblance between the Gulf and the oligotrophic waters of the open sea in terms of their physical, chemical and biological properties. The Chl-total dissolved nitrogen-renewal rate regression model was the most significant, accounting for 60% of the variation observed in Chl. Predicted distributions of Chl for various combinations of the independent variables, based on Bayesian analysis of the models, enabled comparison of the outcomes of specific scenarios of interest as well as further analysis of the system dynamics. The present statistical approach can be used as a methodological tool for testing the resilience of coastal ecosystems under alternative managerial schemes and levels of exogenous nutrient loading.

Developing an optimal sampling design: A case study in a coastal marine ecosystem

The development of a sampling design for optimisingsampling site locations collected from a coastalmarine environment has been the purpose of the presentwork; application of statistical analysis and spatialautocorrelation methods have been carried out. Thedataset included data collected from 34 sampling sitesspaced out in the Strait of Lesbos, Greece, arrangedin a 1×1 NM grid. The coastal shallow ecosystem wassubdivided into three zones, an inner one (7stations), a middle one (16 stations) and an offshorezone (11 stations). The standard error of thechlorophyll-a concentrations in each zone hasbeen used as the criterion for the sampling designoptimisation, resulting into reallocation of thesampling sites into the three zones. The positions ofthe reallocated stations have been assessed byestimation of the spatial heterogeneity and anisotropyof chlorophyll-a concentrations usingvariograms. Study of the variance of the initialdataset of the inner zone taking into account spatialheterogeneity, revealed two different sub-areas andtherefore, the number of the inner stations has beenreassessed. The proposed methodology eliminates thenumber of sampling sites and maximises the informationof spatial data from marine ecosystems. It isdescribed as a step-by-step procedure and could bewidely applied in sampling design concerning coastalpollution problems.

Application of the lognormal equation to assess phytoplankton community structural changes induced by marine eutrophication

A methodological approach was developed for the quantification of the structural changes of phytoplankton communities induced by marine eutrophication. The lognormal equation assigning species abundance to doubling intervals (octaves) of individuals formed the basis of the proposed methodology and the field validation process was based on phytoplankton enumeration and classification data characteristic of eutrophic, mesotrophic and oligotrophic waters. Five octave sets with different sizes were tested for goodness of fit against field data and the set with the smallest size of doubling intervals was selected for further consideration. The application of the lognormal equation was evaluated statistically with field data and it was considered satisfactory at the 87% level. The changes in the shape of the lognormal equation induced by eutrophication were expressed by three characteristic parameters of the equation: the number of the modal octave, the number of species in the modal octave, and the shaping factor. Significant differences were observed for the three parameters among eutrophic, mesotrophic, and oligotrophic waters; the number of the modal octave was high in eutrophic and mesotrophic waters, the number of species in the modal octave has shown a trend of low values under mesotrophic conditions and the shaping factor has shown a considerable increase from eutrophic to oligotrophic waters.

A niche-based modeling approach to phytoplankton community assembly rules.

Six niche-based models proposed by Tokeshi, based on different assumptions of resource allocation by species, were fitted on phytoplankton relative abundance distributions, and potential environmental and biotic factors supporting the applicability of the fitted models were discussed. Overall 16 assemblages corresponding to different sampling times, various environmental conditions, and resource regimes within a year were fitted to the models. Phytoplankton biovolume was used as a measure of abundance, and a randomization test was applied to compare the model fit to the field data. The majority of the phytoplankton assemblages (11 of 16) were successfully described by the Random Fraction model, which is based on the theoretical assumption that resource is apportioned by the species in a random way. Only a few assemblages (three of 16), characterized by extremes in resource availability or disturbance, were not fitted by any of the models. The Random Fraction model in particular was rejected due to a steep slope during the first ranks, while the rest of the distribution remained relatively even, providing further evidence of resilience in phytoplankton communities. Although larger cells seem to have the potential to develop higher biomass, it seems that other factors, including the surface-to-volume ratio, counterbalance this advantage, resulting in a random-like behaviour in resource acquisition by phytoplankton, irrespective of cell size or species identity.

Aquatic microcosms: a methodological approach for the quantification of eutrophication processes

Aquatic microcosm systems were evaluated as a tool for thequantitative description of phytoplankton, bacteria andnutrient relationshipsand the study of nutrient cycling and eutrophication. Thecriteria for the self-developing microcosms were their relative complexity compared to the naturalsystem and the replicability of similar treatments. Theobjective of the work wasto establish standardized procedures for their design,initiation, methodologyand modeling. Glass aquaria were used, each containing 30liters of seawater anda number of inorganic nutrient enrichments was attempted,based on medium ’f‘.The ’f/40‘ dilution treatment was found to be the mostsuccessful on the basisof system complexity and the replication between treatments.The quantitativedescription of the flow of energy through the microbialfood-web was performedusing a simulation model. The relatively high ecosystemcomplexity, thesatisfactory reproducibility and the goodness-of-fit of thedata to the simulation model indicated that the naturally derived aquatic microcosms enriched with medium ’f/40‘ can be used for the study of eutrophication processesconcerning nutrient enrichment and phytoplankton –; bacteriacycling in the coastal marine environment.