Assessing responses of the Hiroshima Bay ecosystem to increasing or decreasing phosphorus and nitrogen inputs

• Sensitivity analyses to assess the ecosystem’s responses of Hiroshima Bay were conducted.
• Different N and P load levels were applied for the sensitivity analyses.
• P had a significantly greater impact on the primary production than N.
• 25% increase of P load increased the primary production but led to N-limitation.
• 25% simultaneous increase of P and N could bring the ecosystem back to eutrophic state.

The Japanese Government is seeking an appropriate level of nutrient load from the land to maintain the highest possible estuarine fishery production and water transparency simultaneously. To provide a scientific basis for the governmental inquiry, we conducted sensitivity analyses using an ecosystem model of Hiroshima Bay in order to assess the ecosystem’s responses to phosphorus and nitrogen inputs. Load levels of phosphorus (Case P), nitrogen (Case N) and both phosphorus and nitrogen (Case NP) that were different from the average loading recorded during 1991–2000 (±25%, ±50%, and ±75%) were applied. The results showed that phosphorus had a significantly greater impact on the primary production of the bay than nitrogen. Case P+25 increased the primary production but led to N-limitation. However, it was found that Case NP at the levels over +25% could bring the Hiroshima Bay ecosystem back to its eutrophic state of 30 years ago.