No abstract

Preventing harmful algal blooms (HABs) is needed to protect lakes and downstream ecosystems. Traditionally, reducing phosphorus (P) inputs was the prescribed solution for lakes, based on the assumption that P universally limits HAB formation. Reduction of P inputs has decreased HABs in many lakes, but was not successful in others. Thus, the "P-only" paradigm is overgeneralized. Whole-lake experiments indicate that HABs are often stimulated more by combined P and nitrogen (N) enrichment rather than N or P alone, indicating that the dynamics of both nutrients are important for HAB control. The changing paradigm from P-only to consideration of dual nutrient control is supported by studies indicating that (1) biological N fixation cannot always meet lake ecosystem N needs, and (2) that anthropogenic N and P loading has increased dramatically in recent decades. Sediment P accumulation supports long-term internal loading, while N may escape via denitrification, leading to perpetual N deficits. Hence, controlling both N and P inputs will help control HABs in some lakes and also reduce N export to downstream N-sensitive ecosystems. Managers should consider whether balanced control of N and P will most effectively reduce HABs along the freshwater-marine continuum.

Mitigating the global expansion of cyanobacterial harmful blooms (CyanoHABs) is a major challenge facing researchers and resource managers. A variety of traditional (e.g., nutrient load reduction) and experimental (e.g., artificial mixing and flushing, omnivorous fish removal) approaches have been used to reduce bloom occurrences. Managers now face the additional effects of climate change on watershed hydrologic and nutrient loading dynamics, lake and estuary temperature, mixing regime, internal nutrient dynamics, and other factors. Those changes favor CyanoHABs over other phytoplankton and could influence the efficacy of control measures. Virtually all mitigation strategies are influenced by climate changes, which may require setting new nutrient input reduction targets and establishing nutrient-bloom thresholds for impacted waters. Physical-forcing mitigation techniques, such as flushing and artificial mixing, will need adjustments to deal with the ramifications of climate change. Here, we examine the suite of current mitigation strategies and the potential options for adapting and optimizing them in a world facing increasing human population pressure and climate change.

We conducted continuous-flow experiments on intact sediment cores from Laguna Madre, Sabine Lake, East Matagorda Bay, and Nueces Estuary to evaluate internal nitrogen (N) sources, sinks, and retention mechanisms in Texas estuaries having different salinities. Mean ammonium (NH ) flux ranged from slight uptake (negative values) Nitrogen (N), a key nutrient limiting primary production in coastal ecosystems, can be introduced to coastal waters via tributaries, atmospheric deposition, or groundwater inputs as nitrates (NO or NO ), ammonium (NH ), and or-ganic N, or through N-fixation (Brock 2001;Paerl et al. 2002;Seitzinger et al. 2002). Available N is removed from coastal waters via physical transport, sediment burial, or conversion to gaseous forms, such as N 2 or N 2 O, via denitrification (Seitzinger et al. 1984;Brock 2001). An improved understanding of N dynamics in coastal systems is needed 1 Corresponding author (gardner@utmsi.utexas.edu). 2 Current address: Pusan National University, 30 Jangjeon-dong, Geumjeong-gu, Busan 609-735, Korea. AcknowledgmentsRick Kalke, Matt First, Melody Bernot, and Walter Dodds provided assistance for some of the field collections. Discussions and suggestions about N dynamics in Texas coastal regions provided by Ron Benner, Paul Montagna, and other colleagues are appreciated. We are grateful for the thorough and constructive observations and suggestions provided by two anonymous reviewers.