Profile: Phosphorus management
BayCEER director Dr Stefan Peiffer discusses novel perspectives in the management of phosphorus in aquatic ecosystems, which require novel research approaches.
The element phosphorus displays two faces in the environment. On the one hand, phosphorus is essential and not substitutable for all life on Earth. On the other, phosphorus can cause significant problems in aquatic ecosystems receiving high loads of nutrients, leading to eutrophication (i.e. adverse biological effects) and a severe deterioration of the water quality. Although marked progress had been made to remove phosphorus particularly from urban wastewater streams, diffuse phosphorus input, e.g. through erosion from agricultural land or incomplete sewage treatment in remote areas, still occurs, hence conflicting with the EU Water Framework Directive (WFD) objective of a good ecological status.
Our common perspective on eutrophication and water quality problems related to phosphorus is shaped by the relationship between phosphorus loading of an aquatic system and its ecological response, i.e. algal biomass (the Vollenweider model). Despite the continuous and costly efforts on controlling eutrophication worldwide, this relationship still includes considerable unexplained variability. Aquatic systems exist in which high loads of phosphorus do not stimulate biomass growth and vice versa. Apparently, key factors contributing to biogeochemical cycling of phosphorus have been overlooked.
A consortium of researchers from the European Science Foundation (ESF) funded Research Networking Programme FIMIN (Functionality of Iron Minerals in the Environment) is therefore calling for a new perspective in the management of phosphorus in the receiving water bodies.
Coupling of element cycles
It is commonly accepted that phosphorus mobility in aqueous systems is closely linked to the biogeochemistry of iron minerals due to the strong affinity to iron oxides and the formation of iron-phosphorus minerals. Hence, phosphorus mobility is a function of all those factors that also control the dissolution and transformation of iron minerals, i.e. the linkage to the element cycles of carbon and sulphur. Through these interactions phosphorus mobility in bottom sediments of the receiving water bodies (such as the Baltic Sea) is related to the larger scale processes within the upstream catchments.
Here, fluxes of phosphorus associated with iron, sulphur and carbon are generated in close relationship to the type of land use and its management in a catchment as well as its geochemical and hydrological properties. Typically, agriculturally used soils are the source of both phosphorus and iron to be transported into the receiving aquatic ecosystems. The interplay between iron mineralogy and the cycles of iron, sulphur and carbon, including a catchment perspective, has not been discussed as a controlling factor for phosphorus mobility in the scientific literature, although the linkage is evident.
Research gaps
Closing this knowledge gap will be the clue for successfully managing aquatic ecosystems connected to agricultural systems and achieving the good ecological and environmental status required by the WFD and EU Marine Strategy Framework Directive. To date, there is a huge deficit as to how the functionality of iron in the environment affects phosphorus cycling in natural and manmade systems, let alone that water protection programmes or agri-environmental programmes would exist that account for such interactions. Hence, research in this field also opens a new perspective on technical innovations, such as recycling of phosphorus from waste streams and/or retention of phosphorus in rural and peripheral settlements.
The novelty in FIMIN’s approach is that phosphorus mobility in receiving water bodies reflects the connectivity between different ecosystems across different scales. This approach requires a complete change of perspective. Rather than looking at single hydrological units (e.g. soils or lakes) within catchments in regard to their capacity to retain or mobilise phosphorus, it is the interconnectivity between these elements that had been overlooked in current research activities. FIMIN’s focus aims to link processes across scales via coupled element cycles. The novelty of our approach is to demonstrate that the ecological state of aquatic systems is significantly dependent not only on the flux of phosphorus, but also on fluxes of catchment born terminal electron acceptors, particularly ferric (trivalent) iron and sulphate, and of organic carbon as the electron donor, modifying element cycles and indirectly regulating the availability of nutrients.
It has been demonstrated that a change in the flux of terminal electron acceptors alone (i.e. decrease in iron, increase in sulfate) has triggered the release of phosphorus from sediment and deteriorated the water quality without any increase in nutrient loading up to a point of a regime shift. In this context one needs to classify catchments in terms of their interplay of iron, carbon and sulfate. As an example of this ‘three-dimensional matrix’, let us consider aquatic systems receiving a low amount of reactive iron, i.e. ferric iron that can be used by bacteria for the decomposition of organic carbon. In such a system, sulfate reduction is promoted over iron reduction, which blocks the iron cycling in sediments resulting in phosphorus release. However, it is still possible for phosphorus to be retained efficiently in the presence of sulfate, provided the amount of iron is in excess to microbially available carbon. Under these conditions sufficient retention capacity for phosphorus is still available.
Sulfate-rich systems include brackish and marine waters as well as water bodies receiving sulfate from cultivated fields, urban and rural areas and mines. The occurrence of ferric iron may thus promote phosphorus retention and reduce eutrophication, whereas sulfate may set apart the coupled iron and phosphorus cycling and enhance eutrophication. The outcome is, however, largely controlled by the amount and microbial availability of organic carbon, formed in the water body itself (autochthonous and typically bioavailable carbon) or coming from the catchment (allochthonous carbon). Additionally, a feedback exists between bioavailability of phosphorus and the formation of organic carbon and there is little if any knowledge on the significance of either autochthonous or allochthonous (riverine and atmospheric) loading of organic phosphorus into aquatic systems.
Unravelling the complex
This approach poses a number of research questions, the answering of which is a requirement to unravel the complex controls on phosphorus mobility in aquatic systems and to make use of this knowledge in designing novel strategies for non-classical phosphorus removal:
- Erosion, strongly accelerated by human actions, as the key process for phosphorus input into aquatic systems delivers iron minerals of different reactivity that are being transported into aquatic ecosystems. The fraction of reactive iron is a function not only of climate, and of geology, but also of plant activity. Natural catchment characteristics and hydrology as modified by human actions govern the transport of different forms of iron, sulphur, carbon and phosphorus from terrestrial to aquatic systems. Hence, there will be significant differences in iron reactivity among catchments and climate on a European scale; and
- The mobility of phosphorus in receiving water bodies reflects the balance between fluxes of sulfate, reactive iron and organic carbon both from the terrestrial environment and through autochthonous production. Hence phosphorus mobility will be controlled by: i) the balance between carbon turnover processes; ii) iron mineralogy; and iii) phosphorus speciation and distribution between inorganic and organic phosphorus.
Ecosystem approach to phosphorus management
As a part of the implementation of EU WFD, programmes of measures (PoMs) are outlined in the WFD that bear a risk of being inefficient, if focusing only on the reduction of nutrient loading and not considering the embedding of nutrient dynamics in the larger scale element cycles. The lack of a conceptual framework has implications on agri-environmental abatement measures.
Yet, most national and international water protection measures and targets focus only on phosphorus and nitrogen (e.g. the Baltic Sea Action Plan of the Helsinki Commission).
They do not recognise the bioavailable fraction of these nutrients, not to mention elements that affect their availability and cycling. Based on our proposed concept, the implementation of the ecosystem-based approach to management of the human pressures is not achievable without understanding of the complex processes produced by iron, sulphur, phosphorus and carbon that can shape the structure of the ecosystem and determine its functional characteristics.
The approach proposed by the FIMIN consortium will provide an additional option to combat eutrophication by controlling iron and sulfate, in the context of varying inputs and bioavailability of carbon, into the aquatic systems.
It will also open the opportunities for new business activities, for instance, in cost effective environmental monitoring and expertise in the spatial planning of land use and utilisation of natural resources.
Dr Stefan Peiffer
Director
Bayreuth Center of Ecology and Environmental Research, BayCEER, University of Bayreuth
+49 (0)92 155 2251
Dr Jouni Lehtoranta
Senior Research Scientist
Marine Research Centre
Finnish Environment Institute (SYKE)
+358 (0)2 9525 1363
