Most of the streams in the Mediterranean region are temporary, following predictable seasonal of flooding and drying, with a transition from lotic conditions to shallow lentic conditions. The goal of our study was to assess the nitrogen and phosphorus dynamics in channel-bed processes of temporary streams between floods. Results show that, during winter, temperatures ranged between 9.5 and 11.2 °C and oxygen concentration ranged from 8.0 to 9.5 mg L?1, whereas, during summer, temperatures varied between 21.2 and 26.8 °C and oxygen between 1.2 and 5.3 mg L?1, with oxygen depletion in the pools during the night. The nitrate concentrations were far more abundant during winter (February), while ammonium concentration increased after stream fragmentation into pools (especially in July when oxygen depletion conditions favoured ammonification). Results on sediment profiles showed that the most active sediment layers for NH4-N are the top 2–3 cm, corresponding to the sediment depositional sites of the stream. Phosphate concentrations had larger variability, yet concentrations decreased from winter to spring and increased again in summer, when the shallow water pools were formed. Sediment profiles at the sediment depositional sites showed that PO4-P was more dynamic in the first 6 cm.
In Mediterranean temporary streams, nutrient dynamics vary seasonally, as the system transits from lotic conditions to shallow lentic conditions, evidencing the regeneration of nutrients from organic and inorganic matter during the flow cessation period.
In running waters, nutrients generated at one location will be naturally transported some distance downstream before subsequent re-utilization, as interdependent processes described as “nutrient spiralling”. Yet, most of the streams constituting the Mediterranean catchments are temporary, becoming either intermittent or ephemeral. Over an annual cycle, these systems expand, contract, and fragment, following sequential, predictable, seasonal events of flooding and drying. These temporary streams provide a large contrast in physical properties, in a similar way to that seen in comparative studies between streams from temperate regions and lakes: At one end, temporary streams are lotic systems with unidirectional flow and sometimes high flushing rate, and at the other end they form small lentic shallow systems. Thus, the nutrient dynamics in temporary streams are mainly determined by the characteristic sequence of dry periods and the following floods.
The dynamics of dissolved inorganic nutrients depend on their being transported from the catchment to the water column and on all the transfer processes linking the water column to the stream bed. Therefore, the main inputs contributing to the nutrient balance and status of these aquatic systems are the external point and diffuse sources plus the internal biogeochemical mineralisation processes. Additionally, the physical and chemical characteristics and the ecology of small Mediterranean streams, while highly complex and dynamic, are strongly influenced by the hydrologic regime. Authors have predicted, for many Mediterranean areas, a rise in the demand for water up to 100% by 2025, which will lead to a major increase in streams’ ephemerality. In addition, ephemerality can also be accentuated by changes in the flow and flood regime induced by climate change. Therefore, it is essential to understand the complexity of physical and chemical changes during the period of flow cessation followed by pool formation. During these dry periods, the shallowness of the water column associated with the low discharge and with late spring/summer high temperatures may enhance nutrient dynamics at the sediment/water column interface. In addition, the importance of benthic mineralisation to the trophic chain increases as the water column gets shallower. Thus, in these spring/summer ephemeral pools, important biogeochemical processes may take place, in combination with water shallowness, high water temperature and low oxygen concentrations.
The objective of the present study was to assess the nitrogen and phosphorus dynamics in channel-bed processes of temporary streams during the period of flow cessation followed by pool formation. Therefore, a selected section of the Pardiela stream, which comprises a depositional zone and an eroding site, was studied between January and August 2004. The Pardiela stream constitutes one of the many small temporary streams in the Mediterranean region that has been studied within the scope of the European project TempQsim (EVK1-CT-2002-00112).
In temporary rivers of Mediterranean catchment areas, hydrodynamic variability has severe implications for the physical and chemical environment. In both situations (lotic and lentic conditions), there was a highly significant positive linear relationship between daily variation in mean water temperature and dissolved oxygen concentration, with higher oxygen concentrations following the increase in the temperature of the water during the day. This daily variation was sequentially more evident during the transition from lotic to lentic conditions, in which the temperature range was much higher, with oxygen depletion during the night. During winter, the oxygen saturation index was higher and with small inter-quartile variation, whilst, in summer, the oxygen saturation index decreased but showed a much higher inter-quartile variation. Results are in agreement with the principle proposed by Odum (1958) in, in which photosynthesis, respiration and aeration are responsible for the diurnal fluctuations of oxygen in streams. However, on a seasonal scale, there was a highly significant negative linear relationship between the mean water temperature and the dissolved oxygen concentration. At the seasonal level, the higher oxygen concentrations and higher oxygen saturation index correspond to the winter lotic conditions and the lower correspond to the summer lentic conditions. These results suggest a higher contribution of the aeration factor during winter, being gradually replaced by biological activity in spring. In summer, corresponding to shallow lentic conditions, the higher oxygen concentrations and higher oxygen saturation index during the day, especially between 12.00 and 15.00, were determined by biological activity. Results show that the catchment may supply much of the dissolved nitrogen and phosphorus to temporary streams but, after flow cessation, most of it may also be internally recycled within the temporary pools through biogeochemical processes. Since phosphorus is mostly transported as particulates and nitrogen often is as well, mineralisation of nutrients is strongly associated with the streambed process and, at the same time, strongly influenced by the hydrology of the system as it determines more depositional zones or eroding sites. In fact, in the Pardiela system during lotic conditions, February and March, total phosphorus (TP) ranged between 1070 and 250 ?g L?1 (median 485 ?g L?1, N=12) and SRP represented 2%–11% of TP (median 8%, N=12). For the same period, total nitrogen (TN) ranged between 4266 and 428 ?g L?1 (median 1408 ?g L?1, N=12) and DIN represented 19%–91% of TN (median 75%, N=12).
The sediment characterization of the Pardiela stream shows that the depositional sites are composed of a higher percentage of smaller grain size particles and a higher percentage of organic matter content and total carbon, especially in the sediment fraction below 2 mm, meaning that the depositional sites have comparatively higher potential for mineralisation processes as the system changes from lotic to lentic conditions. As the efflux of nutrients is a consequence of the primarily heterotrophic metabolism of the sediments, “benthic-pelagic” processes are more closely coupled under shallow lentic conditions. After the sedimentation of particles, dissolved phosphate may be released from the sediment particulate organic matter by P-mineralisation and P-desorption, and thereby supply the efflux of phosphate to the water column. This was observed from the water column data and from the sediment porewater profiles during lentic conditions, in which the temperature range was much higher, undergoing oxygen depletion during the night. Thus, internal P-loading may still persist in a similar manner to that known from shallow lakes showing hysteresis. In addition, the vertical distribution and relative abundance of inorganic nitrogen compounds in the sediment are controlled by the redox state, and ammonification may be enhanced by re-mineralisation of particulate organic matter, while the nitrification process is generally limited by low oxygen concentrations, and ammonia may accumulate. Thus, under anaerobic conditions, ammonium may diffuse upwards to the overlying water, as observed from the water column data and from the sediment porewater profiles during lentic conditions. Moreover, during the transition from lotic to lentic conditions, sediment profiles show that the most active sediment layer for dissolved reactive phosphorus is the top 6 cm, while ammonium-nitrogen is more dynamic in the first 2–3 cm.
The organic content of the sediment can express the potential availability of nutrients, since sediment bacteria are capable of the complete oxidation of a broad range of organic compounds. This may explain the lower phosphorus and nitrogen concentration in the eroding site profiles compared with the higher concentration and more dynamic profiles at the depositional site. Yet, nutrient cycling and transformation occur by both abiotic and biological processes. The significant differences between day and night spring profiles (7 and 27 April, at the depositional site, and 7 April at the eroding site), with higher phosphate concentrations during the night, may result from oxygen depletion due to respiration, which during the day may be compensated by autotrophic oxygen production from epibenthic periphyton . In addition, the reduction in phosphate concentration during the day may also result from biological uptake during the photosynthetic active period . Primary producers, microbes and organic matter form the three primary biological P pools in wetlands, , explaining the higher P dynamics at the depositional site comparative to the eroding site. There were no significant differences between day and night spring profiles of ammonium at the depositional site, probably due to the complexity of factors involved in the nitrogen cycle, especially the central role of bacteria in the transformation from one nitrogen form to another. However, micro-algae and filamentous algae may be responsible for the reduction in concentrations of nitrogen and phosphorus in the first centimetres of sediment depth and in the water column during spring. In fact, these primary producers reach the highest biomass in April/May (Morais et al., unpublished data) contributing to the oxygen depletion during the night period, especially observed after the formation of pools. During summer, the algae biomass decreased, contributing also to the organic enrichment of the lentic systems, in a way similar to that suggested for other aquatic systems.
Results show that, in temporary streams, nutrient dynamics vary seasonally, suggesting that, during the first flow and first flush events following late spring/summer pools, the external source of nutrients into downstream water reservoirs does also include the regeneration of nutrients from organic and inorganic matter during the flow cessation period.
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