A low-tech, low-cost passive sampler for the long-term monitoring of phosphate loads in rivers and streams

The concentration of dissolved reactive phosphorus (DRP) in rivers can change intermittently within minutes depending on the weather and water discharge (Q), or activities in the watershed. Accordingly, accurate estimation of the annual DRP load requires frequent sampling or even continuous monitoring, which is laborious and cost-intensive. We present the design and laboratory evaluation of a new, robust, low-cost, low-tech device based on passive samplers (P-traps). The traps use Fe-(oxy)hydroxide coated quartz sand as an adsorbent enclosed in a vertical grid of individual cells separated from the river water by filter membranes. They are inexpensive, easy to handle, resistant to repeated desiccation and immersion and exposable for several months. They permit estimation of discharge dependent time weighted average DRP concentrations (C-Q relationships) and annual P loads of rivers characterized by highly variable DRP concentrations with a relative accuracy of +/- 3%

Balancing nutrient inputs to Lake Kivu

The primary production in meromictic Lake Kivu is sustained by external nutrient inputs and by internal loading due to upwelling caused by sub-aquatic sources. We present here the results of external loading of phosphorus (P), nitrogen (N) and silica (Si) by rivers and atmospheric deposition measured from 2006 to 2008. These external inputs are compared to internal loading. The input of soluble-reactive P (SRP), supplied in equal parts from rivers and atmospheric deposition, adds up to 230 t P yr(-1), 20 times less than total P load. Ammonium (mainly via rainwater) and nitrate (mainly via rivers) are primary sources of the dissolved N load (5400 t N yr(-1)), with both species contributing similar to 50%. Dissolved Si input (40,000 t Si yr(-1)) is unique in that only similar to 60% enters by rivers, while the remaining similar to 40% comes from sub-aquatic sources and atmospheric deposition is negligible. Based oil the molar nutrient ratios, we identify P as the limiting factor for algae production. Despite the strong anthropogenic impact on the catchment and the high particle erosion (74 t km(-2) yr(-1)), the area-specific nutrient mobilization is rather low. The external nutrient input is therefore not the cause for the reported increase of methane production in the last decades. External loading to the epilimnion plays a lesser role for all three nutrients (similar to 10% for SRP, similar to 25% for dissolved N and similar to 45% for dissolved Si), as compared to the lake-internal loading by upwelling (90%, 75% and 55%, respectively). Lake Kivu, therefore, is similar to other East African large lakes in that the internal loading exceeds the external loading. Despite the substantial uncertainty of the load estimates of up to 50%, we can conclude that the observed nutrient input is consistent with the primary production of 260 g C m(-2) yr(-1) recently measured by Sarmento et al. (2006) and also consistent with the lake-internal fluxes established by Pasche et al. (in press). (C) 2009 Elsevier Inc. All rights reserved