Water management and climate changes increases the phosphorus ...

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194 ( ) V. Claero et al. The Science of the Total Enironment 228 1999 193202 1996 . . Water management is a paramount problem in subarid countries, and the construction of water reservoirs is a frequent management tool. In the early 1980s a dam was built in the upper part of the Palmones River which started to store water in 1987. The normal rainfall in the catchment area Ž averaged for the last 30 years. ranged from 800 to 1050 mm year1 , however, in the last 3 years Ž 19931995. there has been a persistent 1 drought, 718 mm year Ž Carreira et al., 1995 . . As a consequence of both situations two main transformations in the catchment area of the Palmones River have occurred in recent years: Ž. 1 the reduction of the river discharge by the construction of the dam in its upper part and Ž. 2 climatic changes which led to a severe drought which affected the flow of water in the river between the dam and the estuary. The water flow decreased in the estuary from 8 m 3 s 1 in 1987 to 3 1 Ž . 4 m s in 1995 Carreira et al., 1995 . Simultaneously the input of phosphorus increased as a consequence and an increase in the levels of total phosphorus in the sediment should have been expected over the same period. The decrease of freshwater input changed the tidal exchange and the input of phosphate by tidal flux also contributes to increased eutrophication ŽClavero et al., 1997a . . Studies of phosphorus cycling in estuarine sediments have mainly been based on measurements of phosphate or total phosphorus concentrations Ž Lebo and Sharp, 1992 . . However, data on changes of various phosphorus fractions Žcalcium bound to phosphorus and non-calcium Table 1 Comparison of some physiographical characteristics of the estuary over the two time periods considered 2 19871989 19931995 Area of drainage basin Ž km . 95 95 Longitudinal extent of estuary Ž km. 3.5 3.5 Tidal range Ž m. 1.0 1.0 1 Tidal frequency Ž day . 1.67 1.67 2 Estuarine surface Ž km . 4.5 3.75 Mean depth Ž m. 2.0 1.5 Tidal rangemean depth Ž m. 0.5 0.66 1 Rainfall Ž mm year . 8001050 25 3 1 River flow Ž m s . 8 4 Area covered by water Ž over 100% . 100 75 bound phosphorus and organic phosphorus. are scarce in estuarine environments ŽRuttenberg, 1992; Ruttenberg and Berner, 1993 . . In this study, the interannual pattern of variation of the different fractions of phosphorus in two periods Ž A. from 1987 to 1989 and Ž B. from 1993 to 1995 have been studied and will be related to the water management strategies since 1987 and to the climatic changes described above, to predict temporal trends in eutrophication. The aim of this study is to describe the large scale changes observed in the sediment of a small shallow estuary Ž Palmones, Southern Spain. by measurements of the different fractions of phosphorus. 2. Material and methods 2.1. Description of the study area The Palmones River Estuary is located in Southern Spain Ž Fig. 1. at the end of a small catchment area. A comparison of some physiographic characteristics of the estuary during the two time periods considered is shown in Table 1. Palmones is a well mixed shallow estuary Žmaxi- mum depth 3.5 m. with a range of salinity from 29 to 35 Ž Clavero et al., 1997a . . The flow of freshwater in the Palmones Estuary is dominated by the Palmones River. At 23.5 km from the mouth of the estuary is the dam mentioned above, which started to store water in 1987. The results of transformations cited above and the severe drought in the last years, induced dramatic modifications in the system Ž Tables 1 and 2 . . 2.2. Sampling and analytical methods Sediment cores were taken monthly from March 1987 to November 1989 Ž Clavero, 1992. and quarterly from March 1993 to March 1995, using three PVC core tubes Ž 10 cm i.d., 15 cm long. inserted by hand into the sediment, were retrieved and covered in both extremes with silicone tops to minimize the contact with the air and transported to the laboratory in an icebox at 4C. In the laboratory the sediment was extruded from the cores and sliced in segments of 2 cm, under N2
( ) V. Claero et al. The Science of the Total Enironment 228 1999 193202 195 Table 2 Summary of average annual of some chemical and biological variables analyzed over the two time periods considered. P-flux were indicated as positive when released from the sediment to the water and as negative when the P is accumulated into the sediment 19871989 19931995 Water column Phosphate Ž mol l . 2.50.7 8.152.1 Ammonium Ž mol l . 7.82.0 222 Nitrate Ž mol l Sediment . 9.81.3 175 Phosphate-flux 217491 27 to 220 Ž 2 1. Calcium Ž mol l . 300700 25003000 Dissolved phosphate 1 Ž mol l . 165 15035 Organic matter Ž % . 30.8 142 Redox potential Ž mv. 180 220 Inorganic phosphorus 2 Ž IP, g m . 4.51 19070 Organic phosphorus 2 Ž OP, g m . 11.81.2 10749 Total phosphorus 2 Ž TP, g m . Benthic organisms 16.32.3 297119 Zostera noltii Ž g dry wt. m . 100300 0 Ula rotundata Ž g dry wt. m . 0 50200 Hydrobia ulae Ž ind m . 10005000 700 Nereis diersicolor Ž ind m . 340900 0 1 a b 1 c b 1 c b a b mol m day 1 a c a See Clavero Ž 1992 . . b See Carreira et al. Ž 1995 . . c See Clavero et al. Ž 1997b . . d This study. e See Perez-Llorens ´ and Niell Ž 1990 . . a b a d a b a d a d a d 2 e b 2 b b 2 b b 2 a b atmosphere. Each slice was dried at 60C for 24 h and divided in two subsamples. One was used to determine the inorganic phosphorus fractions Ž Williams et al., 1971 . , non-calcium bound phosphorus Ž NAIP. and calcium bound phosphorus Ž AIP . . NAIP was the sum of CDB Žcitrate- dithionite-bicarbonate. and NaOH extractable phosphorus and AIP was extracted with HCl on the residue after the previous treatment. So, the total inorganic phosphorus Ž IP. was considered as the sum of both fractions Ž NAIPAIP . . The second subsample was used to determine the organic phosphorus Ž OP. by extraction with HClNaOH Ž Sommers et al., 1972 . . Soluble phosphate was determined before and after the digestion and the difference represented the con- Ž . centration of OP Sommers and Nelson, 1972 . All the analysis of phosphate were made by means of the malachite green method ŽFernandez ´ et al., 1985. using a Technicon AAII. 2.3. Statistical analysis Ž . Two-way Analysis of Variance 0.05 were used to compare the profiles in depth and time of different fractions of phosphorus. Previously the data were tested for normality using the G-test. 3. Results Ž 2 The concentrations of AIP g m . over the two periods of study are shown in Fig. 2. In the period A values ranged between 1.25 and 2.50 g m 2 . Maximum values were recorded in spring 1989 in the surface of the sediment. During period B, the trend of AIP to increase in the sediment is clear: 40 g m 2 in March 1993 rising to 100 g m 2 in March 1995. In Fig. 3 the time series of NAIP concentrations are presented. The values during the period A ranged between 2.0 and 3.0 g m 2 with maximum values at the end of 1987 and at the beginning of 1988. No values obtained during the period A exceeded 3 g m 2 . During B the concentration increased from 50 to 200 g m 2 . The depth profile of this fraction showed a gradient with significant maximum values at the surface. The OP was three to four times more abundant than the IP fraction in the sediment during A Ž . 2 Fig. 4 and ranged between 10 and 14 g m . Though the concentration of OP during B reached 50200 g m2 , an increase up to 15-times, OP was less than the IP fraction Ž Fig. 5. during this period. The variations of the NAIP, AIP and OP with time and depth were analyzed Ž two way ANOVA . , separately for each period. The results Ž Table 3. showed that in both periods NAIP and OP concentrations significantly increased with time and depth, whereas only time trends of increase are significant for AIP values indicating a homogeneous depth distribution. 3.1. Integrated alues of NAIP, AIP and OP in sediment. The depth-integrated P concentrations was used
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