Ground water and surface water interactions are of fundamental importance for the biogeochemical processes governing phosphorus (P) dynamics in riparian buffers. The four most important conceptual hydrological pathways for P losses from and P retention in riparian buffers are reviewed in this paper: (i) The diffuse flow path with ground water flow through the riparian aquifer, (ii) the overland flow path across the riparian buffer with water coming from adjacent agricultural fields, (iii) irrigation of the riparian buffer with tile drainage water from agricultural fields where disconnected tile drains irrigate the riparian buffer, and (iv) inundation of the riparian buffer (floodplain) with river water during short or longer periods. We have examined how the different flow paths in the riparian buffer influence P retention mechanisms theoretically and from empirical evidence. The different hydrological flow paths determine where and how water-borne P compounds meet and interact with iron and aluminum oxides or other minerals in the geochemical cycling of P in the complex and dynamic environment that constitutes a riparian buffer. The main physical process in the riparian buffer-sedimentation-is active along several flow paths and may account for P retention rates of up to 128 kg P ha(-1) yr(-1), while plant uptake may temporarily immobilize up to 15 kg P ha(-1) yr(-1). Retention of dissolved P in riparian buffers is not as pronounced as retention of particulate P and is often below 0.5 kg P ha(-1) yr(-1). Several studies show significant release of dissolved P (i.e., up to 8 kg P ha(-1) yr(-1)).

Phosphorus Retention in Riparian Buffers: Review of Their Efficiency

SUMMARY. 

1
This review considers the internal fluxes and transformations of nitrogen and phosphorus in wetland ecosystems. Emphasis is placed on the dynamic nature of nutrient cycling and the review is slanted towards an applied perspective, namely the possible use of wetlands as sinks for unwanted nutrients.

2
A number of basic concepts pertaining to wetland ecosystems are first explained. These are: successional time scales, exchange equilibria and the concepts of storage and through flow, resource consumption and supply including the ideas of new and regenerated nutrients and the nutrient spiralling concept. Much of the following review material is referenced back to these concepts.

3
Descriptions of the basic pathways of nutrients through different types of wetland systems are given with the emphasis placed on the movement into and out of the major storage compartments of wetland systems.

4
The problems of conversion of qualitative information (or data in concentration units) on nutrient movements and transformations, into data on mass flows are then discussed. The importance of understanding groundwater, evapotranspiration processes and the effects of floods and seasonality on mass flow calculations can be significant. Unidentified groundwater sources can dilute nutrient concentrations, and evapotranspiration can increase concentrations. The pattern of through flow can also alter nutrient levels. Increasing residence time has the effect of decreasing nutrients in the wetland outflow.

5
The review then considers the effects of adding nutrients to wetlands. The concept of the loading capacity is discussed in relation to the length of time a wetland can continue to remove nutrients from through flow. Sediment accumulation and degassing are seen as the major long-term nutrient sinks. Nutrient enrichment results in biological changes to wetlands. These involve both changes in species composition and productivity. Not all are deleterious.

6
The literature indicates that natural wetlands are not particularly effective as nutrient sinks when compared with conventional stripping plants but their value lies in removal of diffuse nutrient runoff at low concentrations. Dealing with this type of runoff by conventional means is not generally feasible.

7
Recent studies show that artificially created wetlands can he effective systems for nutrient (particularly N) removal only if their internal removed mechanisms are understood and if these are optimized by management techniques.

Cycling and retention of nitrogen and phosphorus in wetlands: a theoretical and applied perspective

Phosphorus and nitrogen mass balances of five wetlands (two beaver ponds, two conifer-Sphagnum swamps and one sedge fen) situated in three catchments in central Ontario, Canada, were measured. Monthly and annual input-output budgets of total phosphorus (TP), total nitrogen (TN), total organic nitrogen (TON), total inorganic nitrogen (TIN), ammonium ion (NH4
+ -N), nitrate (NO
 3
 −
 -N) and dissolved organic carbon (DOC) were estimated for the five wetlands during the 1982–83 and 1983–84 water years. Except for the deepest beaver pond (3.2 m) which had annual TP retention of −44% (−0.030 ± 0.015 g m−2 yr−1), the wetlands retained < 0.001 to 0.015 g M−2 yr−1 ; however, this wasless than 20% of the inputs and the estimated budget uncertainties were equal to or greater than the retention rates. Annual TN retentions ranged from −0.44 to 0.56 g m−2 yr−1 (−12 to 4%) but were not significantly different from zero. The wetlands transformed nitrogen by retaining TIN (16 to 80% RT) and exporting an equivalent amount as TON (−7 to 102% RT). The beaver ponds, however, retained NO
 3
 −
 while NH
 4
 +
 was passed through or the outputs exceeded the inputs. In contrast, the conifer swamps retained both NH
 4
 +
 and NO
 3
 −
 . DOC fluxes into and out of the beaver ponds were equal (−18 and 4% RT) but output from the conifer swamps exceeded input by > 90%. Marked seasonal trends in nutrient retention were observed. Nutrient retention coincided with low stream flow, increased evapotranspiration and biotic uptake during the summer. Net nutrient export occurred during the winter and spring when stream flows were highest and biotic uptake was low.

Phosphorus and nitrogen retention in five Precambrian shield wetlands

Indicators of nutrients transport from agricultural catchments under temperate climate: A review

The effect of land use patterns on river water quality was studied in three different river basins located in Epirus, Northwestern Greece. Studies were conducted from October 2000 to {September} 2001. During this period, the parameters chemical oxygen demand (COD), biological oxygen demand (BOD), NO2−, NO3−, NH4+ and PO43− were measured, employing standard methods of analysis. The results were subjected to principal component analysis (PCA) for the estimation of the underlying variable correlations and were further explored by means of cluster analysis. The values of the above parameters were also compared with those awkward in the Fresh Water Fisheries Directive (78/659/EEC). It was found that the phosphate content was much higher than the upper limiting criteria for eutrophication for salmonid waters, whereas nitrate levels were lower than the permissible criteria according to the Nitrates Directive 91/676/EEC for drinking water. The inorganic nutrient load was mostly attributed to sites that drain agricultural areas, especially during winter and spring. The organic matter was due to urban activities during autumn.

J. H. Peverly

Papers

Stream Transport of Nutrients Through a Wetland 1