A New Seagrass Map for the Venice Lagoon

Transcription

1 In: Proceedings of the Sixth International Conference on the Mediterranean Coastal Environment, MEDCOAST 03, E. Özhan (Editor), 7-11 October 2003, Ravenna, Italy. Vol 2: A New Seagrass Map for the Venice Lagoon Andrea Rismondo (1), Daniele Curiel (1), Francesco Scarton (1), Daniele Mion (1), Giovanni Caniglia (2) (1) SELC scarl, Via dell Elettricità 5, 30174, Marghera-Venezia, Italy Tel: Fax: rismondo@selc.it (2) Dipartimento di Biologia, Università di Padova, Via Trieste 75, Padova, Italy Tel: Fax giovanni.caniglia@unipd.it Abstract In 1990 the first mapping project of submerged aquatic rooted vegetation was carried out to determinate the coverage of three species of seagrasses (Zostera marina Zostera noltii and Cymodocea nodosa occurring in the Lagoon of Venice. After twelve years a new program of research has been started in the framework of a monitoring project carried by Consorzio Venezia Nuova, on behalf of Magistrato alle Acque, Venice. Part of this project deals with the update of seagrass map of Venice Lagoon. All shallow bottoms, tidal flats and channels (not deeper than 2.5 metre) of the Lagoon have been surveyed, as it was made for the previous mapping of Field operations have been carried out by boat, adopting a satellite positioning system with D- GPS technology (accuracy: 1 metre) connected to a portable PC where all data collected in the field, concerning presence of the species, coverage percentage and other information about seagrass meadows have been recorded and daily downloaded to a desk station.

2 All data have been recorded in a GIS, utilizing ARC-GIS software. Definition of the meadows on the map was made possible with a long and accurate desk work, both by integration of recorded waypoints and by satellite and airplane photo interpretation. C. nodosa extends now in the Lagoon of Venice over 2945 hectares (1634 ha in 1990), Z. marina over 3442 ha (3642 in 1990) and Z. noltii over 633 ha (4141 in 1990). The comparison with 1990 seagrass distribution shows a very clear regression of Z. noltii in the inner areas, while Z. marina has been increasing its coverage in the central basin of the Lagoon, where strong dystrophic events, caused by macroalgae blooms, were frequently recorded until few years ago. C. nodosa does not show a clear trend apart from a strong loss close to Chioggia town (southern basin), due its to artificial removal operated by clam collectors. Introduction In 1990 the first mapping project of submerged aquatic rooted vegetation was carried out to determinate the coverage of three species of seagrasses (Zostera marina Linnaeus, Zostera noltii Hornemann and Cymodocea nodosa (Ucria) Ascherson occurring in the Lagoon of Venice (Caniglia et al. 1990, 1991; Magistrato alle Acque 1991). After twelve years a new program of research has been started in the framework of a monitoring project carried out by Consorzio Venezia Nuova, on behalf of Magistrato alle Acque, Venice. Part of this project consisted in the update of Venice lagoon seagrass map. Field surveys were performed from May to September 2002, followed by data analysis and production of maps presented in this paper. All shallow bottoms, tidal flats and channels (not deeper than 2.5 metres) of the Lagoon have been interested by the survey, as it was made for the previous mapping of 1990, resulting in an high-detailed seagrass map. The principal objective was the production of the seagrass map, so it was possible to make comparison with the previous data. Through this comparison it comes possible to analyse colonising and regressing phenomena relating them with other ecosystem components and functions and their geographical location, with the help of ESRI ArcGIS 8.2 s analysis environment. Materials and methods Mapping procedures took into consideration three species identified in Venice Lagoon: Z. marina, Z. noltii and C. nodosa. Ruppia genus has not been considered, despite its presence in lagoon, because it is restricted to very small sites (usually ponds inside salt marshes) and has a very scattered distribution. Cartographic methodology used to carry out our goals had foreseen a GIS system fitting out, planned to receive future improvements and permitting, what is more important, comparisons with 1990 seagrass map. Final results, consisting on 1:10,000 scale map and based on digital cartographic data, have been carried out both on a paper media sized ISO A1 (594x841mm) and on GIS data files (ESRI shape -.shp - file type).

3 Gis system calibration and data collection First of all the DGPS global positioning system calibration, integrated with GIS software (ESRI ArcGIS 8.2), was carried out, in order to evaluate its precision degree and to find the best way for data saving and transfer to GIS software. Calibration had permitted to set a DGPS system with metric precision in collecting cartographic data. Based on the results of this first phase, a field data card used by operators to collect information, covering and some others data was set up. The aim of this phase was also to obtain a reference coverage scale (Fig. 1). Every field team, equipped with a GIS system described before installed on a small boat, was provided with photos derived from IKONOS images (panchromatic or IR, depending on situations and needs). Mapping procedures Image analysis had highlighted areas with presence or devoid of seagrasses: in the latter operators had make a field check to confirm remote sensing data. In the other areas was made a more accurate field check, collecting cover data and border for every seagrass patch. Relieves was carried out according to vegetation characteristic: along external seagrasses patches borders; along transects to explore areas where irregular seagrass cover make compulsory an integral territorial mapping. For each mapped seagrass patch were collected geographic and topologic information, together with vegetation data (specie and coverage degree), according with the following scale (Fig. 1): Fig. 1: Coverage scale used during field mapping. Patches characterized by a mosaic of two or three species has been classified as mixed population and different coverage values were recorded. Data gathered on the field were used to produce preliminary maps, these maps had been subsequently checked and processed overlapping collected data with remote sensing photos and aerial photos, as well with following field come out, to produce the definitive cartographic data.

4 Cartographic results Using methodologies above described it has produced the final seagrasses map for the Venice Lagoon; this map it is a digital geographic database, created using ESRI ArcGIS 8.2 suite, consisting of a series of overlapping layers, precisely: topographic data; 1990 seagrass map (Magistrato alle Acque 1991) 2002 seagrass map (realized using original data Fig. 2).

5 Fig. 2: Venice Lagoon seagrass map, year 2002.

6 MEDCOAST 03 Fig. 3: C. nodosa distribution in 1990 and 2002.

7 Fig. 4: Z. marina distribution in 1990 and 2002.

8 MEDCOAST 03 Fig. 5: Z. noltii distribution in 1990 and 2002.

9 Results On the basis of data collected and of produced digitalized map, extension of all vegetation patches has been calculated. A qualitative and spatial-topological comparison has been performed between present and 1990 data. For this purpose all the 1990 cartographic information (CAD Microstation software) has been transferred to ArcGis 8.2. The comparison is shown in Table 1 where all types of assemblage of vegetation are presented. Observing the data presented in Table 1 and in figures 2-5, it comes clear that a marked modification of the internal structure of population occurred in the years, pointing out a strong reduction of Z. noltii beds (pure and mixed). At the same time an surface expansion trend can be observed for both C. nodosa and for Z. marina, for the latter especially in Central lagoon. Overall, for the whole lagoon, we calculated a retreat of seagrass beds extension of 62 ha only (-1.1%). Nevertheless, in this general picture of stability, there is a high local variability, with a number of more or less emphasized variations in extension, especially for mixed meadows, which indicates an evident dynamism. For this reason a separate analysis for each basin (North, Central and South) is more effective and it helps general comprehension of current trends. Table 1: Comparison between 1990 and 2002 species coverage Variations pure C. nodosa beds pure Z. marina beds pure Z. noltii beds mixed Z. marina C. nodosa beds mixed Z. noltii C. nodosa beds mixed Z. noltii Z. marina beds mixed Z. noltii Z. marina C. nodosa beds Total Northern lagoon Northern lagoon showed larger retreats in comparison with 1990 data (Table 2), with loss of 584 ha of meadows. Higher losses are referred to the disappearance of Z. noltii population (Fig. 5), which accounted for 88 % of the total loss. This evidence indicates a most likely decrease in environmental quality in comparison with 1990.

10 Table 2: Comparison between 1990 and 2002 taxa coverage in Northern lagoon Variations pure C. nodosa beds pure Z. marina beds pure Z. noltii beds mixed Z. marina C. nodosa beds mixed Z. noltii C. nodosa beds mixed Z. noltii Z. marina beds mixed Z. noltii Z. marina C. nodosa beds Total Central lagoon Here we found the most evident increase, leading to the hypothesis of a increased water quality (Table 3). We found an increase in Z. marina distribution which amounts to 747 ha, with high coverage percentage. This species is responsible for 97 % of the whole increase in this basin. A similar trend has been observed for Z. noltii (Fig. 5) e C. nodosa (Fig. 3) with an increase of, respectively, 88 and 60 ha. Table 3: Comparison between 1990 and 2002 species coverage in Central lagoon Variations pure C. nodosa beds pure Z. marina beds pure Z. noltii beds mixed Z. marina C. nodosa beds mixed Z. noltii C. nodosa beds mixed Z. noltii Z. marina beds mixed Z. noltii Z. marina C. nodosa beds Total Southern lagoon All three species showed distribution changes in comparison to 1990, even if these are less marked than those referring to the Central lagoon (Table 4). Net losses amount to 237 ha. Z. noltii (Fig. 5) registered a loss of 727 ha for pure population and of 2253 ha for mixed populations (a total loss of 2980 ha). Concerning Z. marina (Fig. 4), we observed a retreat of 924 ha. It is important to note that a strong difference between pure and mixed meadows extension occurred, being increasing the former (1294 ha) and regressing the latter (-2218 ha). About C. nodosa (Fig. 3), we observed an increase of 1202 ha. Only few limited meadows showed retreats in comparison with 1990 map.

11 Table 4: Comparison between 1990 and 2002 species coverage in Southern lagoon Variations Pure C. nodosa beds Pure Z. marina beds Pure Z. noltii beds mixed Z. marina C. nodosa beds mixed Z. noltii C. nodosa beds mixed Z. noltii Z. marina beds mixed Z. noltii Z. marina C. nodosa beds Total Discussion and conclusions We observed different trends in Venice Lagoon seagrass meadows. Z. noltii showed the strong retreat, more emphasized in Southern basin; the other two species showing overlapping movements. The most significant finding is the retreat of Z. noltii, from mixed meadows with Z. marina, in open shallow bottoms and from tidal flats of the whole lagoon. A general overview of species dynamics points out that the differences in meadows extension between 1990 and 2002, about 62 ha, can not be considered significant considering the whole lagoon. Locally, we believe the disturbance caused by clam collectors, especially in Southern lagoon, who eliminate the leaf canopy and the root compartment with a special floating device is one of the most important threat to seagrass meadows. Only in Southern basin this artificial loss amounts to 240 ha, mostly C. nodosa beds. Concerning the strong loss of Z. noltii, both with pure and mixed populations, this phenomenon has not a clear explanation. Nowadays, a large surface of bare tidal flats is now present around salt marshes, in Southern and Northern lagoon, where this species was abundant in the past. Causes of this loss will be investigated taking into account the known variations in the environmental parameters most likely to influence species occurrence, such as water temperature, turbidity, bottom grain size, etc. References Caniglia, G., Borella, S., Curiel, D., Nascimbeni, P., Paloschi, F., Rismondo, A., Scarton, F., Tagliapietra, D. and Zanella, L. (1990), Cartografia della distribuzione delle fanerogame marine nella Laguna di Venezia, Giorn. Bot. Ital., 124(1), 212. Caniglia, G., Borella, S., Curiel, D., Nascimbeni, P., Paloschi, F., Rismondo, A., Scarton, F., Tagliapietra and D., Zanella, L. (1991), Cartografia computerizzata delle fanerogame marine nella Laguna di Venezia, Bollettino dell A.I.C., 81, 1-4.

12 Magistrato alle Acque (1991), Nuovi interventi per la salvaguardia di Venezia. Composizione delle comunità biologiche. 1 a Fase. Rilievi sui popolamenti delle barene ed aree circostanti e sulla vegetazione dei bassifondi, Studio A Rapporto Finale. Consorzio Venezia Nuova, unpublished report. Keywords: seagrasses, Zostera marina, Zostera noltii, Cymodocea nodosa, Lagoon of Venice, mapping, GIS