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Doñana's water quality, in danger due to intensive agriculture and a poor residual water treatment

Irene Paredes, researcher of the study

Eutrophication is a major cause of wetland degradation worldwide. In recent decades, reductions in nutrient inputs have led to improvements in water quality in many rivers and lakes in central and northern Europe, but long-term trends are less clear in southern Europe. The Doñana Biological Station conducted the first comprehensive study of water quality in Doñana, one of the most important wetland complexes in Europe and the Mediterranean region.

The core area of Doñana is a large shallow, seasonal marsh (UNESCO World Heritage Site—WHS) that floods during rainy, cool winter months, then dries out during the summer. The marsh is fed by three main streams whose catchments are within a Biosphere Reserve but are impacted by greenhouses (for cultivating fruit, irrigated with groundwater), poorly treated urban wastewaters and tourism.

From 2013 to 2016, the research team monitored nutrient and phytoplankton chlorophyll-a (chla) concentrations in surface waters of the Doñana marsh and the three main streams. They quantified changes in greenhouse cover since 1995 using satellite images. Nutrient concentrations in streams were consistently higher than in the marsh, particularly in the Partido and Rocina streams that regularly reached concentrations equivalent to a "bad physico-chemical status" under the EU Water Framework Directive (WFD), and whose catchments suffered a fivefold expansion of greenhouses from 1995 to 2016.

The Partido was the most polluted stream, and the most affected by wastewater effluents, and had particularly high concentrations of NH4+ and NO2? across seasons. Patterns in chla concentrations were less consistent, but streams generally had higher concentrations than the marsh. Nutrient concentrations in spot samples within the marsh largely depended on a combination of evaporation (as revealed by higher stable isotope ?2H values in the water column) and spatial processes (concentrations increase close to stream entry points, where conductivity is lower).

Anthropogenic nutrient pollution in entry streams is a serious problem in Doñana, with extensive stretches too toxic for fish. Reinforcement of policies aimed at reducing nutrient inputs to Doñana are urgently required to meet the biodiversity conservation and environmental objectives for the WHS and WFD, respectively. Paradoxically, the marsh is currently relied upon to purify the water entering from streams.



Paredes, I., Ramírez, F., Aragonés, D., Bravo, M.A., G. Forero, M., Green, A.J. (2021). Ongoing anthropogenic eutrophication of the catchment area threatens the Doñana World Heritage Site (South-west Spain). Wetlands Ecology and Management.

Read the full press release (Spanish)

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From landfills to lakes: gulls as transporters of nutrients

From landfills to lakes: gulls as transporters of nutrients

The lesser black backed gull Larus fuscus is now the second most abundant wintering waterbird in Andalusia, and has increased in numbers in recent years. Fuente de Piedra, the biggest shallow lake in Andalusia and a Ramsar site famous for its flamingo colony, is the principal gulls roosting site in midwinter. Gulls are the most important source of nitrogen and phosphorus inputs to the lake during winter through their guano, and feed in four landfills up to 80 km away in Málaga and Córdoba provinces according to GPS data. This study has quantified the amount of nutrients gulls import to Fuente de Piedra lake. The obtained results provide a vital step to understand the effects of waterbirds in so called "guanotrophication". Gulls take advantage of easy pickings at landfills for feeding, so they can transport an important amount external of nutrients to the lake, where they spend a large proportion of the day. GPS data were used to determine at which landfills gulls were feeding, and the relative importance of each. Although gulls are regularly counted at the lake, gulls are early risers and GPS data showed that most of them had already flown off towards landfills when counting was carried out, and this allowed correcting the counts for missing birds. Movement data were also used to estimate the amount of daytime gulls spent at the lake over different winters, which determines how much of their guano is deposited in the lake. Laboratory analyses of faeces and pellets completed the calculations of nutrient inputs. As estimated in this study, gull contribution seems to be the most important nutrient source in winter with about 10 kg of nitrogen per Ha and 2 kg of phosphorus per Ha. This may bring implications within the ecosystem, whose state seems already eutrophied. Gull excreta is rich in nitrogen and phosphorus, which combined with other nutrient sources such as flamingos, run-off or inputs from the town's Wastewater Treatment Plant, may lead to an excess of nutrients in the lake. There are signs this has caused effects such as the loss of higher plants and a boom in algae. The water levels of Fuente de Piedra vary a lot from one year to another, and low water levels may increase the impact of gull guano. The study found that gulls preferred to spend more time in the lake when water levels were around 30 cm, when islands were available for roosting, and this can lead to increases in nutrient concentrations. This is the first time GPS data is combined with other methods to determine the origin and quantities of nutrients into aquatic systems, and it is the first step to understand the connection between anthropogenic landfills with aquatic bodies. informacion[at] Martín-Vélez et al (2019). Quantifying nutrient inputs by gulls to a fluctuating lake aided by movement ecology methods. Freshwater Biol