In natural seagrass systems, regular oscillatory motion -like that caused by surface wind-waves -enhances uptake of dissolved inorganic nitrogen (DIN) relative to current-driven, unidirectional flows. A mobile field flume was deployed to measure the uptake of DIN by intact shallow seagrass communities exposed to unidirectional, oscillatory, and combined flows. The flume volume was spiked with 15 N-labeled DIN to measure nutrient uptake rate constants for the entire system (S) and to determine height-specific nutrient uptake rates of seagrass epiphytes (ρ). In oscillatory and combined flows, S depended positively on the water speed, but also inversely on a modified Keulegan-Carpenter number (KC). This ratio compares the wave-orbital excursion distance to canopy element spacing. Experiments characterized by relatively low KC values were associated with significantly higher uptake efficiencies than experiments characterized by high KC values. Uptake efficiencies for high KC conditions were similar to those measured in comparable unidirectional flows. Measured canopy flow attenuation was also found to increase with KC, a result in line with expectations from a parameterized conceptual model. Damping variability alone could not explain the observed oscillatory flow uptake enhancement however, a result that seemingly highlights the role of both element flexural response and unsteady boundary layer mechanics in the exchange process. In all flow conditions, DIN uptake rates of epiphytes harvested from the bottom of the blades (ρ bot ) were lower than those for epiphytes harvested from the upper portion of the blades (ρ top ). While uptake by the top epiphytes exhibited flow dependency, uptake by bottom epiphytes did not.

The increasing availability and use of high-frequency water quality sensors has enabled unprecedented observations of temporal variability in water chemistry in aquatic ecosystems. However, we remain limited by the prohibitive costs of these probes to explore spatial variability in natural systems. To overcome this challenge, we have developed a novel auto-sampler system that sequentially pumps water from up to 12 different sites located within a 12 m radius to a single water quality probe. This system is able to generate high temporal frequency in situ water chemistry data from multiple replicated units during experiments as well as multiple sites and depths within natural aquatic ecosystems. Thus, with one water quality probe, we are able to observe rapid changes in water chemistry concentrations over time and space. Here, we describe the coupled multiplexer-probe system and its performance in two case studies: a mesocosm experiment examining the effects of water current velocity on nitrogen dynamics in constructed wetland sediment cores and a whole-ecosystem manipulation of redox conditions in a reservoir. In both lotic and lentic case studies, we observed minute-scale changes in nutrient concentrations, which provide new insight on the variability of biogeochemical processes. Moreover, in the reservoir, we were able to measure rapid changes in metal concentrations, in addition to those of nutrients, in response to changes in redox. Consequently, we believe that this coupled system holds great promise for measuring biogeochemical fluxes in a diverse suite of aquatic ecosystems and experiments.

Abstract. An automated multiplexed pumping system (MPS) for high-frequency water chemistry measurements at multiple locations previously showed the ability to increase spatial and temporal data resolution and improve understanding of biogeochemical processes in aquatic environments and at the land–water interface. The design of the previous system precludes its use in volume-limited applications in which highly frequent measurements requiring a large sample volume would significantly affect observed processes. A small-volume MPS was designed to minimize the sample volume while still providing high-frequency data. The system was tested for cross-contamination between multiple sources, and two applications of the technology are reported. Cross-contamination from multiple sources was shown to be negligible when using recommended procedures. Short-circuiting of flow in a bioreactor was directly observed using high-frequency porewater sampling in a well network, and the small-volume MPS showed high seasonal and spatial variability of nitrate removal in stream sediments, enhancing data collected from in situ mesocosms. The results show it is possible to obtain high-frequency data in volume-limited applications. The technology is most promising at the reach or transect scale for observing porewater solute dynamics over daily timescales, with data intervals  < 1 h for up to 12 locations.

Abstract. An automated multiplexed pumping system (MPS) for high frequency water chemistry measurements at multiple locations was previously reported. This technology showed potential to increase spatial and temporal resolution of data and improve our understanding of biogeochemical processes in aquatic environments and at the land-water interface. The design of the previous system precludes its use in volume-limited applications where highly frequent measurements requiring large sample volume would significantly affect observed processes. A small volume MPS was designed to minimize sample volume while still providing high frequency data. The system was tested for cross contamination between multiple sources and two applications of the technology are reported. Cross contamination from multiple sources was shown to be negligible when using recommended procedures. Short-circuiting of flow in a bioreactor was directly observed using high frequency porewater sampling in a well network, and the small volume MPS showed high seasonal and spatial variability of nitrate removal in stream sediments, enhancing data collected from in situ mesocosms. The results show it is possible to obtain high frequency data in volume-limited applications. The technology is most promising for observing pore water solute dynamics and improving existing solute transport models for saturated or partially saturated soils and media.

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