Nutrient Dynamics

Various factors can limit the rate of primary production if demand (requirements of autotrophs) exceeds availability. Limiting factors in streams are typically light and

Figure 5 Comparison of algal colonization in zones of upwelling and downwelling following floods in Sycamore Creek, AZ. Valet HM, Fisher SG, Grimm NP, Camill P (1994) Vertical hydrologic exchange and ecological stability of a desert stream ecosystem. Ecology 75(2): 548-560.

Days after flooding

Figure 5 Comparison of algal colonization in zones of upwelling and downwelling following floods in Sycamore Creek, AZ. Valet HM, Fisher SG, Grimm NP, Camill P (1994) Vertical hydrologic exchange and ecological stability of a desert stream ecosystem. Ecology 75(2): 548-560.

nutrients. Because desert streams often have open canopies and receive abundant light, nutrients are the primary constraint on algal growth. Lack of precipitation in arid and semiarid regions leads to very slow rates of weathering of parent materials, which can lead to phosphorus (P) limitation as rocks are the ultimate source of P in ecosystems. However, in many well-studied arid and semiarid watersheds of the US southwest, volcanic-derived parent materials yield highly dissolved P. Primary production in these desert streams is thus limited by nitrogen (N).

Nutrients enter streams via inputs from upstream, from groundwater and overland flow, in plant materials deposited from the riparian zone, and in the case of N, via fixation of atmospheric N2 by cyanobacteria. Unidirectional flow of water results in continual input and output of nutrients in dissolved and particulate forms, although inputs of limiting nutrients may be low due to processing that occurred upstream. Nutrient spir-aling theory, a set of hypotheses that describe how nutrients move between water column, subsurface, and biotic compartments while being transported downstream, predicts that nutrient uptake should be more efficient under conditions of nutrient limitation. In streams limited by N, for example, inorganic N is rapidly removed from the water column by biota. For hot desert streams, rates of nutrient uptake can be particularly rapid due to high temperatures and light availability, which increase rates of biological reactions. Concurrent with rapid uptake by algae, excretion of inorganic N by invertebrate consumers can represent nearly 30% of the total N delivered to the ecosystem. Over successional time, export of rafting algal mats or stranding of algae on the stream banks during dry periods results in loss of organic N from the stream ecosystem. Budgets of organic N for desert streams thus conform with the successional trajectory of terrestrial ecosystems wherein ecosystems at late successional stages tend to lose nutrients. In contrast to terrestrial ecosystems, however, net primary productivity may continue to be positive during late successional stages of desert streams, resulting in continued uptake of inorganic nutrients by primary producers.

In surface water, nutrient cycling is dominated by uptake of nutrients by algae and benthic biofilms. The dominant pathway of nutrients in the surface is therefore from inorganic to organic forms. Regeneration (mineralization) of inorganic nutrients in the subsurface may in turn resupply dissolved inorganic nutrients. Processes occurring in the hyporheic and parafluvial zones thus contribute strongly to patterns of nutrient availability in desert streams. Water flowing through sediments slows in velocity allowing for greater interactions between sediment surfaces and materials delivered in water. Microbes inhabiting the interstitial areas of sediments transform nutrients present in these downwelling zones, influencing the spatial distribution of nutrients in the stream channel.

In coarse sediments where dissolved oxygen remains relatively high as water moves through the hyporheic zone, mineralization often dominates N transformations, resulting in a localized increase in streamwater dissolved inorganic N concentrations at locations of upwelling. Increased nutrient availability in zones of upwelling is often associated with hot spots of algal biomass. These patterns are typical of alluvium-dominated reaches where algae are the predominant primary producers. In patches where macrophytes colonize gravel bars and parafluvial zones or in patches of fine sediment deposition, dissolved oxygen concentration in the subsurface is decreased due to root respiration and decomposition of plant-derived organic matter, and hyporheic flows are slowed, all of which lead to hypoxic or anoxic conditions. Hot spots of denitrification are associated with anoxic conditions in the hyporheic zone and water upwelling downstream of such patches is therefore depleted of inorganic N (Figure 6).

Because of these differences in nutrient processing between surface and subsurface flowpaths, streams that undergo drying may exhibit marked spatial variability in nutrient availability. Sections of the stream that dry may continue to harbor subsurface flows and rapid transformation of nutrients for some time after surface flows are depleted. Nutrient inputs and outputs from dry reaches may show strong contrasts in forms or concentration of nutrients. In contrast, reaches characterized by perennial flow tend to show dampened upstream-downstream contrasts due to the homogenizing effects of processes occurring in surface flows.

As with nearly all aquatic ecosystems, the surrounding terrestrial landscape influences nutrient dynamics in desert streams. In deserts, however, hydrologic connectivity between the stream and terrestrial portions of the catchment, including the riparian zone, are variable in

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Site along flowpath

Figure 6 Changes in concentration of nitrate in water flowing through plant-colonized gravel bars (top). Note precipitous drop in concentration when water encounters a plant patch (denoted by branch figure). Denitrification is a likely mechanism accounting for the drop in concentration of nitrate; in situ rates of denitrification increase in plant patches (bottom). Redrawn from Schade JD, Fisher SG, Grimm NB, and Seddon JA (2001) The influence of a riparian shrub on nitrogen cycling in a Sonoran desert stream. Ecology 82: 3363-3376.

time. Deposited nutrients and those stored by plants and microbes may accumulate in the riparian zone and uplands during dry periods. When precipitation or snow-melt events occur, water carries these particulate and dissolved nutrients overland from the uplands to the stream, and between the riparian subsurface and the surface stream. This creates pulses of nutrient transport between desert streams and their watersheds. Pulsed inputs of nutrients result in hot moments of nutrient processing, short time periods with rapid rates ofnutrient transformations. Hot moments may account for a significant fraction of annual nutrient processing within riparian zones of desert streams.

Connectivity between terrestrial and aquatic portions of desert stream-riparian corridors may also occur from the stream to the riparian zone. Riparian plants can access water and nutrients from the hyporheic zone as well as from shallow groundwater. Access to these more permanent sources of water and nutrients leads to high productivity in riparian zones relative to desert uplands. Stream biota may transfer nutrients between streams and riparian zones of desert streams. Owing to high rates of primary productivity, insect emergence from desert streams can result in significant exports of nutrients out of the wetted stream. Emerging aquatic insects may thus provide a significant source of nutrients to riparian food webs.

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