Chapter 53 described how Earth’s topography affects water depth, creating gradients in light, temperature, water pressure, and nutrient availability. The main primary producers in aquatic ecosystems—phytoplankton and algae—are most limited by light and nutrients along continental margins (coastal zones) and in the surface waters of oceans and lakes (photic zones) (see Figure 53.9C and D). Oceanic NPP peaks at mid-latitudes and along the continental margins (see Figure 57.3B) because these are regions where nutrient concentrations from ocean upwelling are high. Higher NPP is associated with estuaries as well, where nutrients can be “outwelled” from watershed runoff to coastal zones.

In aquatic systems, NPP is limited primarily by iron, nitrogen, and phosphorus. The relative importance of these three nutrients varies depending on the aquatic ecosystem. In much of the open ocean, NPP is limited primarily by nitrogen. But in some locations, particularly the equatorial Pacific Ocean where excess nitrogen has been detected in surface waters, NPP by phytoplankton may be more limited by iron. In a series of experiments in the mid-1990s, biological oceanographers added iron sulfite to the surface waters off the Galapágos Islands and in the Southern Ocean to test the hypothesis that oceanic NPP, and thus the potential of the ocean to take up CO₂ and combat global warming and ocean acidification (see p. 1239), was limited primarily by iron. The experiments showed that NPP was as much as four- to tenfold higher in the plume of water that received the iron additions. While the experiments show that iron limitation is a major factor in reducing the ability of the ocean to absorb CO₂ from the atmosphere, large-scale fertilization of the oceans is impractical at best, and may be somewhat counteracted by respiration of CO₂ by zooplankton and bacteria feeding on phytoplankton.

In lake ecosystems, NPP is limited primarily by phosphorus. The effect of phosphorus on lake NPP was first studied in connection with the declining water quality of lakes in North America and Europe. Wastewater and sewage were implicated in a dramatic increase in eutrophication, a process of ecosystem change initiated by an increase in nutrients consumed by phytoplankton, cyanobacteria, and algae in aquatic systems. Eutrophication can result in the explosive growth of algae (algal blooms). Decomposition of large volumes of dead algae by bacteria can in turn severely deplete oxygen needed by aquatic life (hypoxia). To understand which nutrients were most important to the blooms, David Schindler and colleagues conducted whole lake fertilization experiments, starting in 1969, in a set of small lakes in Ontario, Canada. They added nitrogen, carbon, and phosphorus to all or half of several lakes and found that phosphorus was the major trigger for phytoplankton and cyanobacteria blooms (Figure 57.7). These experiments set in motion a campaign to reduce phosphorus in wastewater by recommending the banning of phosphates in detergents and stripping them chemically in sewage treatment plants.

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