Ρόλος της ατμοσφαιρικής εναπόθεσης θρεπτικών συστατικών και ιχνοστοιχείων στην παραγωγικότητα της νότιο-ανατολικής Μεσογείου

Abstract

Atmospheric deposition of various Nitrogen and Phosphorus species was studied in bulk deposition samples simultaneously collected at several locations around the Mediterranean, during one year period (June 2001-May 2002). Dissolved Inorganic Phosphorus (DIP) and Nitrogen (DIN) atmospheric deposition fluxes ranged from 252 to 608μmol m⁻²y⁻¹ and from 18.1 to 77.9mmol m⁻²y⁻¹ respectively, presenting an important spatial variability within the basin. Wet deposition was found to be the main factor controlling DIN deposition in the Mediterranean. The amount of DIN deposited during the wet period was 2–8 times higher than that deposited during the dry season. It was estimated that about 65% of the total DIP was deposited during the wet period. Dust events as well as regional biomass burning and air mass origin were also found to contribute significantly to the DIP deposition. A significant percentage of the Total Dissolved Phosphorus (TDP) of the samples was in organic form with Dissolved Organic Phosphorus (DOP) accounting for 38% of TDP. DIN/DIP molar ratio of the bulk deposition varied depending on the location of the sampling site in the Mediterranean basin, presenting an increasing trend from the Western (50) to the Eastern Mediterranean basin (170). This variation is similar to that observed in the seawater column, indicating an important link between atmospheric deposition and seawater productivity of the area. The various phosphorus species and their sources in the Mediterranean atmosphere where examined via extended time series of aerosol samples. Total Inorganic Phosphorus (TIP) varied temporarily in the Eastern Mediterranean presenting maximum values during summer and minimum during winter. One fourth of the total TIP was of crustal origin. DIP appeared to follow a similar temporal variation with TIP all over the Mediterranean atmosphere, while its main component had anthropogenic origin (DIPanthr). DIPanthr correlated well with nss-SO₄⁻² indicating thus a way about how this phosphorus species could be included in seawater productivity models. During summer periods DIPanthr correlated well with nss-K⁺. This observation suggests that the atmosphere is enriched with phosphorus during events of biomass burning. The next step towards a better understanding of the role of atmospheric deposition for the seawaters productivity is to clarify the fate of the insoluble compounds in the water column. Thus during a seven years period, 200 sediment samples have been collected at two different depths. Based on the fluxes and the seasonality of various anthropogenic and teriggenious element which constitute the settling material in the water column the following transfer pattern was suggested for the Eastern Mediterranean Basin: The rapid vertical mass transfer which is observed during spring, can be attributed to nutrient abundance which leads to the formation of big aggregates, which settle quite fast. During summer and autumn mass transfer is hindered due to the stratification of the water and the low primary production. During winter any mass transfer in the Eastern Mediterranean area is difficult to be traced due to the very low mass content of the water and the lack of atmospheric dust deposition. The correlation of the atmospheric and the seawater data led to interesting conclusions about the interactions between the two elements. The inert part of the atmospheric dust was found to be transferred almost quantitatively from the surface towards the deeper water layers, verifying the model of vertical mass transfer. The amount of DIN deposited via atmosphere was more than sufficient for the needs of the Cretan seawater ecosystem. In contrast DIP deposition did not follow the same pattern. Thus taking into account that the specific ecosystem develops according the Redfield N/P ratio, it can be deduced that despite the increased DIN deposition the lack of atmospheric DIP determines the seawater productivity. Even though atmosphere seems to have a very important influence on the material that is collected with the sediment traps, it does not explain completely the productivity pattern of the Eastern Mediterranean Basin. Atmospheric matter can be the driving force which will increase biological activity in a water column, as long as it is combined with the appropriate hydrological and meteorological conditions. Overall it became obvious that atmospheric deposition increases the size of the aggregates which are created in the water column during periods of intense biological activity and thus contributes to their rapid settling. This observation should be taken into account in seawater productivity and mass transfer models, for a more complete approach.

Η παρούσα μελέτη στοχεύει να διερευνήσει το ρόλο της ατμοσφαιρικής εναπόθεσης των διαλυτών μορφών θρεπτικών συστατικών (αζώτου και φωσφόρου) στην Μεσογειακή λεκάνη. Σε συνδυασμό δε με την μελέτη των ατμοσφαιρικών μορφών και πηγών φωσφόρου θα προκύψουν ενδιαφέρουσες πληροφορίες για τον κύκλο του φωσφόρου και την επίδραση της ατμόσφαιρας στην παραγωγικότητα ολιγοτροφικών οικοσυστημάτων (Μεσόγειος θάλασσα). Επιπλέον εξετάζει το γίγνεσθαι μιας ποικιλίας στοιχείων εντός της στήλης του νερού προκειμένου να συσχετίσει θαλάσσια και ατμοσφαιρικά δεδομένα και να εξετάσει την όποια αλληλεπίδραση μεταξύ ατμόσφαιρας και θάλασσας. Η όμοια συμπεριφορά του λόγου εναπόθεσης DIN/DIP με τον θαλάσσιο λόγο Ν/Ρ, οδηγεί στο συμπέρασμα ότι η ατμόσφαιρα επηρεάζει ανομοιόμορφα την παραγωγικότητα της Μεσογείου. Βάσει των δεδομένων από τις ιζηματοπαγίδες προέκυψε ένα σενάριο κάθετης μεταφοράς ύλης για την Ανατολική Μεσόγειο. Κύριο χαρακτηριστικό του το ότι την άνοιξη η μεταφορά ύλης συσχετίζεται με την βιολογική δραστηριότητα, τα επεισόδια σκόνης και την δημιουργία μεγάλου μεγέθους σωματιδίων, που ταξιδεύουν γρήγορα προς τα κάτω. Αντίθετα το χειμώνα εξαιτίας της χαμηλής παραγωγικότητας και της ηπιότερης εναπόθεσης ατμοσφαιρικής σκόνης, η θαλάσσια ροή μάζας παραμένει σε χαμηλά επίπεδα. Η καλή εποχική συσχέτιση μεταξύ των ατμοσφαιρικών και θαλάσσιων δεδομένων καθιστά προφανές το ότι η ατμοσφαιρική εναπόθεση συμβάλει στην αύξηση του μεγέθους των συσσωματωμάτων που δημιουργούνται στην στήλη του νερού τις περιόδους με έντονη βιολογική δραστηριότητα, οδηγώντας στην ταχύτατη κάθετη μεταφορά τους. Προκειμένου λοιπόν τα μοντέλα παραγωγικότητας και μεταφοράς στην στήλη του νερού να θεωρηθούν πιο ολοκληρωμένα θα πρέπει να συμπεριλάβουν και την παρατηρούμενη σύζευξη μεταξύ ατμόσφαιρας και θάλασσας.