The Impact of Trawling on Nutrient and Oxygen Fluxes

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1 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας 943 The Impact of Trawling on Nutrient and Oxygen Fluxes in the Cretan Sea C.J. Smith 1, K.-N. Papadopoulou 1, E. Dafnomili 2 & S. Zivanovic 2, 1. Institute of Marine Biological Resources, Hellenic Centre for Marine Research, P.O. Box 2214, 713, Iraklion, Crete, Greece 2. Institute of Marine Biology and Genetics, Hellenic Centre for Marine Research, P.O. Box 2214, 713, Iraklion, Crete, Greece Abstract The impact of otter trawling on sedimentary nutrient and oxygen fluxes were investigated in the oligotrophic Cretan sea. During two seasons (winter and summer) replicate (6) large cores were taken from the field from a commercial trawl ground and an adjacent control area. The cores were incubated under controlled conditions and flux rates estimated over a period of 6-12 days. Flux rates of phosphate, ammonia and oxygen were all positive (absorbed by the sediment) in the study area in both summer and winter, whilst silicate and nitrate were negative (released from the sediment) and nitrite mixed. In the winter, silicate and nitrate flux rates were negative whilst ammonia and oxygen were positive. Trawling did appear to have a significant impact on fluxes with: a) more release of silicate in the winter (open trawling season), b) less release of nitrate in summer (closed trawling season), c) higher absorbance of nitrite in winter, d) higher absorbance of ammonia in summer, e) lower oxygen absorbance in summer. Fluxes are discussed in relation to the impact of trawling on the biota and consequence impacts on fluxes themselves. Κeywords: trawling impacts, nutrient flux, oxygen flux, Cretan Sea, Mediterranean Οι επιπτωσεις της αλιειας με τρατα στη ροη θρεπτικων και οξυγονου στο Κρητικο Πελαγος C.J. Smith 1, K.-N. Παπαδοπούλου 1, E. Δαφνομήλη 2 & S. Zivanovic 2, 1. Ινστιτούτο Θαλάσσιων Βιολογικών Πόρων, Ελληνικό Κέντρο Θαλασσίων Ερευνών 2. Ινστιτούτο Θαλάσσιας Βιολογίας και Γενετικής, Ελληνικό Κέντρο Θαλασσίων Ερευνών Περιληψη Η εργασία αυτή μελετάει τις επιπτώσεις της αλιείας με τράτα στη ροή θρεπτικών και οξυγόνου. Πραγματοποιήθηκαν 2 δειγματοληψίες (χειμώνας & καλοκαίρι) σε 4 σταθμούς στο ολιγοτροφικό Κρητικό Πέλαγος, 2 σε αλιευτικό πεδίο και 2 σε περιοχή μάρτυρα. Οι corers (6 μεγάλοι/σταθμό) διατηρήθηκαν σε μεσόκοσμους υπό ελεγχόμενες συνθήκες για 6-12 ημέρες για αντίστοιχες ημερήσιες δειγματοληψίες. Η ροή των PO 4, NH 4 και O 2 ήταν θετική (απορρόφηση από το ίζημα) και τις 2 εποχές, η ροή των Si 2 και NO 3 ήταν αρνητική (απώλεια από το ίζημα) και των NO 2 μικτή. Το χειμώνα η ροή των Si 2 και NO 3 ήταν αρνητική ενώ της NH 4 και O 2 ήταν θετική. Η αλιεία με τράτα είχε σημαντική επίδραση στη ροή θρεπτικών και οξυγόνου με α) περισσότερη απελευθέρωση Si 2 τον χειμώνα (περίοδο ανοικτή στην αλιεία), β) λιγότερη απελευθερωση NO 3 το καλοκαίρι (περίοδο κλειστή στην αλιεία), γ) μεγαλύτερη απορρόφηση NO 2 το χειμώνα, δ) μεγαλύτερη απορρόφηση NH 4 το καλοκαίρι και ε) χαμηλότερη απορρόφηση O 2 το καλοκαίρι. Τα αποτελέσματα αυτά συζητούνται σε σχέση με τις επιπτώσεις της αλιείας στους βενθικούς οργανισμούς. Λέξεις κλειδια: επιπτώσεις αλιείας, ροή θρεπτικών, ροή οξυγόνου, Κρητικό Πέλαγος, Μεσόγειος

2 944 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας Introduction In northern European waters, a large number of studies have been undertaken on the interactions of trawling and the environment, investigating impacts on many components of the environment (reviewed in Hall, 1999, Kaiser and de Groot 2). The physical impact of trawling can be very complex, ranging from surface topography changes to effects on biogeochemical processes (Pilskaln et al. 1998, Kaiser et al. 22). Latterly investigations have started to focus on ecosystem functionality and processes. One area of concern has been how the reduction in the numbers of bioturbators may effect sediment irrigation and associated processes. Once the large bioturbators are removed and irrigation processes are also reduced, deeper sediments will become less well oxygenated thus reducing the space for other infauna to occupy. In the Eastern Mediterranean a number of studies have been undertaken on faunal or physical impacts (Simboura et al. 1998, Smith et al. 2, Smith et al. 23), with less work on chemical or process impacts. This current work, part of the EU funded COST-IMPACT was part of a study investigating the relationships between fauna, bioturbation, nutrient cycling and trawling impacts. The work presented here refers to seasonal nutrient and oxygen flux experiments carried out in the southern Aegean in and adjacent to a commercial trawling lane, both in the trawling season (October to May) and during the closed season (June-September). Materials and Methods The full area covered by commercial trawling in Iraklion Bay (Island of Crete, southern Aegean) is shown in Figure 1. One of the main trawling lanes, with coverage of 2% per year, follows the 2 m contour and narrows with the contours behind Dia Island (Fig. 1). The sediments were characterised by predominantly silt (85-9%) with smaller fractions of sand and clay. Four areas were selected for bottom sampling, two control areas to the south of the trawling lane (SO) and two areas in the trawling lane (FL). Filed sampling was undertaken during two periods, July/August 22 (non-trawling season) and February/March 23 (trawling period). From each sampling area, two multicore samples (diameter 1 cm, penetration at least 2 cm) were taken, one of which was stored in a cool box for use in the nutrient flux experiment and the other was used for nutrient analysis of the overlying water and organic carbon and chlorophyll analysis of the water and sediments. Figure 1. Sampling sites in the Bay of Iraklion, FL: fishing lane east, SO control area.

3 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας 945 Figure 2. Mesocosm set-up (12 channel pump, 24 stirring heads). The mesocosm system consisted of a cold seawater recycling system (15 deg. C) with water flowing from the chiller through a reservoir tank into the core incubation tank (Figure 2). Each core tube in the incubation tank was fitted with a special head containing an electrical stirring motor (turning speed approximately 1 revolution per seconds). A 12 channel peristaltic pump fed a constant flow (approximately 1 ml per minute) of water from the reservoir tank to the cores. The cores overflowed into the incubation tank. Water samples (2 ml) were taken from each of the cores and the reservoir tank at the same time each day after collection and oxygen concentration and temperature were also measured. The water samples were analysed for phosphate, silicate, nitrate, nitrite and ammonium. The winter experiment was run for 6 days and the summer experiment for 12 days. At the end of each experiment the cores were sieved through a.5 mm mesh to analyse the macrofauna present. Nutrient flux was estimated from the equation: F x = (C i - C o ). Q / A Where: F x =flux of nutrient x micromol. m -2. h - 1 ), C I = mean concentration of x in the header tank (µm), C o =mean concentration of x in the overlying water (µm), Q =flow of water through the core (l. h -1 ), A =area of the core (m 2 ) Results The calculated mean nutrient flux rates for the summer and winter experiments are shown in Figure 3. Flux rates are given as micromole per square metre per hour with standard error. Positive values indicate absorbance by the sediment, negative values indicate sediment release. Mean rates were estimated from the 6 replicates from each sampling area for each of the incubation days. The major results are as follows: Phosphate indicated a very low, but positive flux in both summer and winter. There were no differences between the trawl and control areas within or between seasons. Silicate indicated negative fluxes during both seasons. There was a highly significant difference due to season (2-way ANOVA, P<.1) with more negative fluxes in summer. There were more negative fluxes in the winter in the trawling lane (ANOVA, P<.1) Nitrate, very similar to silicate, indicated negative fluxes during both seasons. There was a highly significant difference due to season (2-way ANOVA, P=.33) with higher negative fluxes in summer. In summer there were significantly lower negative fluxes in the trawl area (ANOVA, P=.5). There was also a significant interaction between season and area.

4 946 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας Phosphate Nitrite Silicate Ammonia Nitrate Oxygen Figure 3. Mean nutrient flux rates (mean and standard error bars) from winter (shaded) and summer flux incubations. Flux rates in micromole per square metre per hour (FL: fishing lane, SO: control). Nitrite fluxes were low (close to zero) and positive with the exception of the control sites in winter. There was a significant difference due to season (2-way ANOVA, P<.31), with greater fluxes in summer and a significant difference between areas as mentioned above with lower higher fluxes in the trawl lane in winter. All Ammonia fluxes were positive. There were significant differences between the sample areas and seasons and the interaction between area and season. Higher fluxes were found in the western areas in winter and higher fluxes were noted in the fishing lane in the summer. Oxygen fluxes were all positive. There were significant differences within seasons for ANOVA (summer P=.21, winter P<.1) with generally higher values in winter between areas. In summer there were significantly lower fluxes in the fishing lane. There was also a significant interaction between area and season. All data values for species, abundance and biomass for the core macrofauna were very low. There were no significant differences in ANOVA tests for species, abundance and biomass within season for the different areas For the 2-way ANOVA there was a significant difference for species between season (P=.49) and for area (P=.24), with perhaps richer species number in the Easterly areas. There was also a significant difference in abundance between seasons (P=.22). Discussion Differences in flux rates between the sampling areas were expected due to seasonal changes (seasonal changes in temperature regimes and water column mixing). This was evident oxygen and for all nutrients except for phosphorus (which may be related to the general very low phosphate concentrations in this oligotrophic area). Differences found in the trawling lane included: a) more negative flux (release) of silicate in the winter (open trawling season), b) less negative flux of nitrate in summer (closed trawling season), d) higher nitrite flux (absorbance) in winter, e) higher ammonia flux in summer, f) lower sediment oxygen flux in summer. Undoubtedly the relationships between physical disturbance, the fauna and fluxes are extremely

5 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας 947 complex. During the trawling season it is easy to understand that the release of silicate increases from a purely physical impact of trawling, i.e. physical turnover of the sediment. As a result of the same physical disturbance the nitrite and ammonia flux increases could be related to more reductive processes acting on exposed reduced sediments. In the summer in the same area the nitrate absorbance and oxygen release will be directly related to one another. This may be due to the lack of direct impact at this time, but a reduction in benthic activity caused by the reduction in fauna in the area. Smith et al. (2) have reported a significant reduction in macrofaunal (species, abundance and biomass) in the trawled area. At a functional level this reduction was primarily in terms of suspension feeders, faunal elements that are responsible for drawing carbon actively down into the sediment. The same authors unpublished data indicate a reduction in the trawled area of large bioturbators (mound and burrow forming species) and it is thought that this may be one of the major reasons for the differences seen in nutrient and oxygen fluxes. Bioturbation activity is an accelerator for many irrigation based processes, such as oxygenation and related aerobic activity as well as detoxification. Widdicombe and Austen (1998) and Widdecombe et al. (24) have demonstrated the role of the large bioturbators in maintaining biodiversity and the modification of sediment chemistry. Many large bioturbators are relatively long-lived species and if removed by trawling may take many years to re-establish. The consequences of their removal is far beyond simple faunal reduction, but has a accelerating impact on the reduction in living space (lowered oxygen penetration) and probable lessened heterogeneity in the habitat (3-D sediment structuring) as well as a reduction in sediment conditioning processes. References Hall, S.J. (1999) The effects of fishing on marine ecosystems and communities. Blackwell Science. Oxford. 296 pp. Kaiser, M. & de Groot, S.J. (eds) (2) The effects of fishing on non-target species and habitats. Biological, conservation and socioeconomic issues. Blackwell Science, Oxford. 399 pp. Kaiser M.J., Collie J.S., Hall S.J., Jennings S. & Poiner I.R. (22). Modification of marine habitats by trawling activities:: prognosis and solutions. Fish and Fisheries 3: Pilskaln C.H., Churchill J.H. & Mayer L.M. (1998). Resuspension of sediment by bottom trawling in the Gulf of Maine and potential geochemical consequences. Conservation Biology 12(6): Simboura, N., Zentos, A., Pancucci-Papadopoulou, M-A., Thessalou-Legaki, M. & Papaspyrou, S. (1998) A baseline study on benthic species distribution in two neighbouring gulfs, with and without access to bottom trawling. P.S.Z.N.: Mar. Ecol., 19(4): Smith, C.J., Papadopoulou, K.-N. & Diliberto, S. (2) Impact of otter trawling on an eastern Mediterranean commercial fishing ground. ICES Journal of Marine Science; 57: Smith, C.J., Rumohr, H., Karakassis I. & Papadopoulou K.-N. (23) Analysing the impact of bottom trawls on sedimentary seabeds with sediment profile imagery. Benthic Dynamics: in situ surveillance of the sediment-water interface. Journal of Experimental Marine Biology and Ecology, : Widdicombe S. & Austen M.C. (1998). Experimental evidence for the role of Brissopsis lyrifera (Forbes, 1841) as a critical species in the maintenance of benthic diversity and the modification of sediment chemistry.

6 948 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας Journal of Experimental Biology and Ecology 228: Widdicombe S., Austen M.C., Kendall M.A., Olsgard F., Schaanning M.T., Dashfield S.L. & Needham H.R. (24). Importance of bioturbators for biodiversity maintenance: indirect effects of fishing disturbance. Marine Ecology Progress Series 257: 1-1.