Hierarchy of factors controls denitrification rates in temperate intermittently closed and open coastal lakes/lagoons (ICOLLS)

Intermittently closed and open lakes/lagoons (ICOLLs) can occur in alternate stable states: clear and turbid, with nitrogen inputs from high-intensity agricultural land use often fuelling phytoplankton growth in ICOLLs. Due to their limited water exchange, ICOLLs are particularly susceptible to eutrophication. In these environments, denitrification may remove a substantial proportion of the land-derived nitrogen load, reducing their vulnerability to eutrophication; however, the factors that influence denitrification in ICOLLs are poorly understood. In this study, we addressed the relative importance of physico-chemical and biotic factors related to nitrate-saturated denitrification rates (including temperature, nutrient/organic matter supply, oxygen conditions, sediment type and benthic macroinvertebrates) in two eutrophic ICOLL ecosystems: one supports some submerged macrophytes, while the other is in a persistent, turbid, phytoplankton-dominated system. Flexible in situ enclosures and denitrification enzyme assay measurements were employed to determine denitrification rates in response to new nitrate pulses, which are commonly observed in these systems. In situ denitrification rates were inhibited in both ICOLLs in winter, whereas in summer they were positively correlated with organic matter availability. Denitrification rates were greater in the shallow, marginal sediments of the ICOLLs. Bioturbating macrofauna significantly enhanced in situ sediment oxygenation and probably transported sediment organic carbon and nitrate simultaneously to sites of denitrification at the sediment oxic-anoxic interface. Our study found that nitrate-saturated sediment denitrification rates were controlled by a hierarchy of temporally and spatially structured physico-chemical and biotic factors in the following order of importance: temperature -> organic matter availability -> water depth -> bioturbation.