Abstract
Whether net primary productivity in an aquatic ecosystem is limited by nitrogen (N), limited by phosphorus (P), or co-limited by N & P is determined by the relative supply of N and P to phytoplankton compared to their elemental requirements for primary production, often characterized by the “Redfield” ratio. The supply of these essential nutrients is affected by both external inputs and biogeochemical processes within the ecosystem. In this paper, we examine external sources of nutrients to aquatic systems and how the balance of N to P inputs influences nutrient limitation. For ocean subtropical gyres, a relatively balanced input of N and P relative to the Redfield ratio from deep ocean sources often leads to near co-limitation by N and P. For lakes, the external nutrient inputs come largely from watershed sources, and we demonstrate that on average the N:P ratio for these inputs across the United States is well above that needed by phytoplankton, which may contribute to P limitation in those lake that experience this average nutrient loading. Watershed inputs are also important for estuaries and coastal marine ecosystems, but ocean sources of nutrients are also significant contributors to overall nutrient loads. The ocean-nutrient sources of N and P are very often at or below the Redfield ratio of 16:1 molar, and can be substantially so, particularly in areas where the continental shelf is wide. This large input of coastal ocean nutrients with a low N:P ratio is one factor that may make N limitation more likely in many coastal marine ecosystems than in lakes.
Highlights
Aquatic scientists have long noted major differences in nutrient control of net primary productivity across different types of ecosystems, with phosphorus (P) limitation in many lakes (Schindler 1974; Vollenweider 1975), nitrogen (N) limitation in many estuaries and coastal marine ecosystems (Ryther 1954), and co-limitation by N and P in many open ocean systems such as the subtropical gyres (Redfield 1958)
Note further than even lakes in largely forested catchments may have inputs of both N and P related to the food and feed term of Net Anthropogenic Nitrogen Inputs (NANI) and Net Anthropogenic Phosphorus Inputs (NAPI), for instance due to nutrient inputs from onsite wastewater treatment systems from nearby homes. This same construct of the N:P ratio for external nutrient inputs setting the stage for limitation of primary production by N vs P should apply to estuaries and coastal marine ecosystems: unless internal biogeochemical processes are sufficient to compensate, nutrient inputs from the watershed with an N:P ratio substantially above the needs of the phytoplankton would be expected to lead to P limitation
Whether an estuary or other coastal marine ecosystem is N or P limited will be determined by the balance of N and P inputs from land, of N and P inputs from ocean sources, and of biogeochemical processes within the ecosystem that alter the availability of N and P
Summary
Aquatic scientists have long noted major differences in nutrient control of net primary productivity across different types of ecosystems, with phosphorus (P) limitation in many lakes (Schindler 1974; Vollenweider 1975), nitrogen (N) limitation in many estuaries and coastal marine ecosystems (Ryther 1954), and co-limitation by N and P in many open ocean systems such as the subtropical gyres (Redfield 1958). Watershed inputs are important for estuaries and coastal marine ecosystems, but ocean sources of nutrients are significant contributors to overall nutrient loads.
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