Response of eutrophication in the eastern Gulf of Finland to nutrient load reduction scenarios

Abstract

The trophic status of the eastern Gulf of Finland, where the largest Baltic metropolis St. Petersburg sits at the mouth of the largest Baltic river Neva, is elevated but existing recommendations on water protection measures are controversial. In this study, the effects of nutrient load reductions on this ecosystem were estimated with the aid of a threedimensional coupled hydrodynamic-biogeochemical model. As a reference, the contemporary seasonal dynamics were simulated with nutrient inputs corresponding to the recent estimates of point and riverine sources. In order to eliminate the effects of natural inter-annual variations, the computations were run under recurrent annual forcing for 3 years, until quasi steady-state seasonal dynamics were reached. Reasonable comparability of simulated concentrations and biogeochemical fluxes to available field estimates provides credibility to scenario simulations. These simulations show that substantial reductions of nutrient point sources in St. Petersburg would affect only the Neva Bay as the immediate receptor of treated sewage waters, where primary production could decrease by up to 20%. Eutrophication in the other parts of the Neva Estuary and in the entire eastern Gulf of Finland would change insignificantly owing to increased nutrient import from the offshore waters. Therefore, more significant changes can occur only via a reduction in nutrient pools in the open Gulf of Finland and the Baltic Proper, which would require a longer time.

Historical changes in primary production in the Seto Inland Sea, Japan, after implementing regulations to control the pollutant loads

“…For example, Harima Nada is next to Osaka Bay and the Kii Channel, and nutrient transport from the Pacific Ocean is also well documented (Yanagi & Ishii, 2004). Sediment may also be a source of nutrients (Savchuk et al, 2009;Jikumaru et al, 2015;Nguyen & Maeda, 2016). In addition, seawater velocity may affect primary production by not only changing the retention of nutrients (Savchuk et al, 2009) but also light conditions (Wetsteyn & Kromkamp, 1994).…”

“…In fact, Yanagi & Ishii (2004) reported that the ratios of the landoriginated and ocean-originated nitrogen and phosphorus loadings to the Seto Inland Sea were 19:81 and 7:18, respectively, indicating that, while the contributions in each subarea may differ, nutrients originating from the open ocean are the main source for the entire Seto Inland Sea rather than land-originated nutrients. In addition, outer coastal areas may respond less sensitively to the reduction of land-originated nutrient loadings than inner coastal areas (Savchuk et al, 2009;Staehr et al, 2017). Therefore, questions about whether primary production has decreased in the Seto Inland Sea and how the reductions in the land-originated nutrient loads to the sea impact productivity remain unanswered.…”