Effects of varying levels of nutrient inputs to coastal marine systems: a case study of a semi-enclosed bay influenced by a large urban population

Port Phillip Bay (PPB) is the largest marine bay on the Australian coast and is the site of Australia’s second largest city, Melbourne. A major environmental study in the 1990s recommended a reduction in the nitrogen (N) input to the bay. Subsequently, improvements to sewage treatment efficiency in the 2000s coincided with the longest and most severe drought in recorded history, resulting in N inputs dropping by more than half in the 2000s compared with the 1990s. Here we review studies conducted over the past 30 years to understand the effects of varying nutrient levels on the ecology of PPB. Studies showed that PPB is an N limited system both in time and space. Biological productivity in PPB was markedly affected by reduced N loads during the drought, resulting in declines in seagrass and kelp cover, as well as benthic fish biomass. Overall, research has shown that while setting conservative limits on N input will effectively negate the risk of widespread eutrophication, there will likely be a tradeoff in reduced bay productivity. Managers will need to consider both sides of this equation when managing the load of N entering PPB, and potentially other marine systems around the world.