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<p>In marine systems, molar ratios of the biolimiting inorganic nutrients, nitrogen (N) and phosphorus (P), are assumed to follow the “Redfield ratio” (N:P = 16:1), which is often used to infer nutrient limitation of phytoplankton biomass. Traditionally, estuaries are considered N-limited (N:P &lt; 16) but with higher coastal N-loading, reports of P-limitation (N:P &gt; 16) have increased. In recent decades, nutrient loading in North Inlet Estuary (NIE) has changed such that dissolved inorganic nitrogen (DIN):dissolved inorganic phosphorus (DIP) ratios increased from ca. 7 to 20 (2002–2021). This suggests that primary production in NIE may be transitioning from N-limitation to NP co-limitation or primary P-limitation. Due to high N:P ratios, we hypothesized that P would be the primary limiting nutrient for phytoplankton production during the summer of 2023. DIN (20&#xa0;µmol&#xa0;l<sup>−1</sup>), low DIP (LP, 5&#xa0;µmol&#xa0;l<sup>−1</sup>), and high DIP (HP, 20&#xa0;µmol&#xa0;l<sup>−1</sup>), combined DIN + LP, or combined DIN + HP were added to water samples collected monthly at Clambank Landing in NIE and incubated in 48-h bioassays. Changes in phytoplankton biomass (chl <i>a</i>) and community composition were measured via high performance liquid chromatography (HPLC) to determine if enrichment indicated limitation by that nutrient. N was the single or primary limiting nutrient for all bioassays, with potential NP co-limitation in June. Our results suggest that, in NIE, DIN:DIP ratios exceeding 200 may still show phytoplankton N-limitation or NP co-limitation. Ambient nutrient concentrations and ratios are an unreliable indicator of nutrient limitation in this estuary and should not be used to infer limiting nutrients.</p>

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Evaluating Potential Changes from Nitrogen to Phosphorus Nutrient Limitation of Phytoplankton Growth in North Inlet Estuary, SC

  • 期刊论文

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  • 发表日期：

    2025年6月25日

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  • Catherine Schlenker,
  • James L. Pinckney

摘要

In marine systems, molar ratios of the biolimiting inorganic nutrients, nitrogen (N) and phosphorus (P), are assumed to follow the “Redfield ratio” (N:P = 16:1), which is often used to infer nutrient limitation of phytoplankton biomass. Traditionally, estuaries are considered N-limited (N:P < 16) but with higher coastal N-loading, reports of P-limitation (N:P > 16) have increased. In recent decades, nutrient loading in North Inlet Estuary (NIE) has changed such that dissolved inorganic nitrogen (DIN):dissolved inorganic phosphorus (DIP) ratios increased from ca. 7 to 20 (2002–2021). This suggests that primary production in NIE may be transitioning from N-limitation to NP co-limitation or primary P-limitation. Due to high N:P ratios, we hypothesized that P would be the primary limiting nutrient for phytoplankton production during the summer of 2023. DIN (20 µmol l−1), low DIP (LP, 5 µmol l−1), and high DIP (HP, 20 µmol l−1), combined DIN + LP, or combined DIN + HP were added to water samples collected monthly at Clambank Landing in NIE and incubated in 48-h bioassays. Changes in phytoplankton biomass (chl a) and community composition were measured via high performance liquid chromatography (HPLC) to determine if enrichment indicated limitation by that nutrient. N was the single or primary limiting nutrient for all bioassays, with potential NP co-limitation in June. Our results suggest that, in NIE, DIN:DIP ratios exceeding 200 may still show phytoplankton N-limitation or NP co-limitation. Ambient nutrient concentrations and ratios are an unreliable indicator of nutrient limitation in this estuary and should not be used to infer limiting nutrients.