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Discussion 48 the sorting of the sand grains and therefore also the permeability of the sediment. As a result the rate of SGD could increase or decrease and the groundwater may even change its route through sediments with a higher permeability. Furthermore discharge is driven by wave setup or set-down (Li et al., 1999) which is changing in time. This means that for instance seawater may recharge the sediment thereby restraining groundwater discharge at the same spot. All this oceanic processes are very variable e.g. wind direction and intensity is changing over time. During the sampling campaign in October 2010 the wind was changing from a light breeze of about level 2 of the Beaufort scale from south west in the beginning of the campaign to west wind with Beaufort numbers 4 to 5 later on. This may therefore have an influence on the variability in SGD. Another observed phenomenon was the formation of methane bubbles in the sediments of Hel beach (Vogler et al., 2011). Those bubbles may have decreased the permeability at times and therefore also contributed to the high variability of SGD at the sampling site. 4.1.3 Extrapolating exercise for SGD SGD may occur throughout the whole Puck Bay. To estimate the influence of SGD seepage rates were extrapolated to the whole Puck Bay area and compared to the surface freshwater input to the Puck Bay (8.1 km 3 a -1 ; (Cyberski, 1993). Puck Bay covers an area of 359.2 km -2 (Bolałek & Graca, 1996). Seepage rates measured in the benthic chambers during October 2010 were extrapolated to one year and multiplied with the whole Puck Bay area. Assuming the largest seepage rate (113 l m -2 d -1 ) the water input via SGD into the whole Puck Bay would be about 14.8 km 3 a -1 (Table 4.2). This would be as much as 65 % of the of the total water input (SGD plus surface runoff). Calculations with the lowest measured seepage rate of 13 l m -2 d -1 would still give 17 % SGD per total inflow of all surface water input and SGD. On a global scale, only about 6 to 10 % of the total freshwater runoff arises from groundwater seepage (Burnett et al., 2003). SGD may have a larger influence on a regional scale like for the Puck Bay area. However, the benthic chamber measurements gave the flux of combined saline and freshwater discharge because of the recirculating seawater component of SGD. The input of fresh SGD into the Puck Bay is therefore expected to be lower than the measured rates. Extrapolation with the highest measured seepage rate may give an idea for the maximal possible input but is not very realistic. Calculations with the lowest measured rate still give relatively high SGD fractions, since there are a lot of areas on the Beach of Hel where no salinity decrease and therefore SGD occurs (Figure 4.1). The six benthic chambers, which only covered a very small area of 0.03 m 2 each on a SGD impacted site, did not give enough
Discussion 49 data to calculate a mean discharge rate for the beach of Hel let alone the whole Puck Bay. Further measurements and combinations with modelling and tracer techniques are needed. Using the estimated mean seepage rate for the Puck Bay of 0.4 l m -2 d -1 (Piekarek-Jankowska, 1994) the SGD portion would only be 1 % of the total water input. Based on that, the influence of SGD on the Puck Bay seems rather low. Table 4.2: Global discharge of freshwater and comparison of the water input via submarine groundwater discharge (SGD) into Puck Bay and via surface freshwater inflow. The total inflow is the sum of the surface run-off and the SGD. Input of SGD was extrapolated to the whole area of Puck Bay (359.2 km 2 ; (Bolałek & Graca, 1996) for different seepage rates. The SGD portion of the total water input is given in %. Seepage rate Input flux SGD per total inflow l m -2 d -1 km 3 a -1 % Reference Global discharge Total runoff 37,400 (E. K. Berner & R. A. Berner, 1987) Fresh groundwater seepage 2,400 6 (Zektser, 2000) Puck Bay mean freshwater inflow 8.1 (Cyberski, 1993) SGD 0.4 0.05 1 calculated from (Piekarek- Jankowska, 1994) SGD in this study 13 1.7 17 calculated from the minimal seepage rate measured in October 2010 in Hel 113 14.8 65 calculated from the maximal seepage rate measured in October 2010 in Hel 4.1.4 DON in the SGD Although the actual water input of SGD to the Puck Bay may be of rather low significance this may not be the case for the nitrogen input. Groundwater mostly contains higher concentrations of organic and inorganic dissolved nitrogen than rivers or precipitation (Johannes, 1980; Mulligan & Charette, 2006). DON compounds in sediment pore waters can be produced as intermediates or end-products of remineralisation or oxidation of sediment organic matter (SOM) (Hatcher & Spiker, 1988; Burdige & Martens, 1990; Amon & Benner, 1996; Burdige & Zheng, 1998). According to that sediments with low organic matter (OM) content should also contain low DON
Page 1 and 2:
Input and fate of dissolved nitroge
Page 3 and 4: Abstract II Abstract The aim of thi
Page 5 and 6: Abstract IV Um die Quellen und Senk
Page 7 and 8: Introduction 1 1 Introduction Eutro
Page 9 and 10: Introduction 3 where one aspect dea
Page 11 and 12: Introduction 5 producer and consume
Page 13 and 14: Material and Methods 7 Sampling too
Page 15 and 16: Material and Methods 9 The main sam
Page 17 and 18: Material and Methods 11 Figure 2.4:
Page 19 and 20: Material and Methods 13 Figure 2.5:
Page 21 and 22: Material and Methods 15 t indicates
Page 23 and 24: Material and Methods 17 f = N N s s
Page 25 and 26: Material and Methods 19 Due to low
Page 27 and 28: Material and Methods 21 f = S P −
Page 29 and 30: Results 23 Table 3.1: Number of sam
Page 31 and 32: Results 25 Figure 3.2: Ammonium (NH
Page 33 and 34: Results 27 positioned closest to th
Page 35 and 36: Results 29 The mixing behaviour of
Page 37 and 38: Results 31 Figure 3.6: Ammonium (NH
Page 39 and 40: Results 33 values found in the seas
Page 41 and 42: Results 35 conservative mixing line
Page 43 and 44: Results 37 3.5 Benthic Chambers To
Page 45 and 46: Results 39 3.6 Core incubation expe
Page 47 and 48: Results 41 δ 13 C values about -18
Page 49 and 50: Results 43 Figure 3.15: Isotope dia
Page 51 and 52: Discussion 45 4 Discussion 4.1 SGD
Page 53: Discussion 47 Table 4.1: Seepage ra
Page 57 and 58: Discussion 51 maxima were approxima
Page 59 and 60: Discussion 53 reduction. This means
Page 61 and 62: Discussion 55 Table 4.3: Different
Page 63 and 64: Discussion 57 Table 4.4: Different
Page 65 and 66: Discussion 59 NH + 4 values. The 18
Page 67 and 68: Discussion 61 ammonium (more than 5
Page 69 and 70: Discussion 63 (McClelland et al., 1
Page 71 and 72: Bibliography 65 Burdige, D.J. & Zhe
Page 73 and 74: Bibliography 67 Jankowska, H., Matc
Page 75 and 76: Bibliography 69 Moore, W.S. (2010)
Page 77 and 78: Bibliography 71 Thamdrup, B. & Dals
Page 79: Selbständigkeitserklärung 73 Selb
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