Seasonal changes in pore water concentrations of nutrients and their diffusive fluxes at the sediment-water interface

Abstract

Seasonal variations of dissolved NH 4 + , NO 3 − , NO 2 − and Si(OH) 4 within an oyster bed sediment from the Bay of Morlaix, Brittany, France, were studied from February 1985 to January 1986. They were compared to the annual pattern of sedimentary organic C and N. Nutrient fluxes at the sediment-water interface were measured using sediment cores and compared to those estimated from solute gradients between pore water and the water column. Organic matter showed three accumulation periods. The highest one occurred during summer. Humic compounds analysis confirmed a marine origin (seaweeds, oyster feces) while they displayed a terrigenous origin in January and April. Vertical profiles of organic matter are characteristic for a strong burial activity. One example of this activity is described through the good correlation which is found between the organic C : N ratios at 1 and 10 cm. Gradients of ammonium and nitrate between the first centimetre of the sediment and the water column ranged from 36.6 to 202.4 and from − 15.2 to 130.2 μmol · 1 −1 · cm −1. For nitrite and silicate, they ranged from 0.8 to 9.2 and from - 3.6 to 152.4 μmol · 1 −1 · cm −1. Whatever the organic matter source was, the degradation pattern in the sediment followed the same processes. Sedimentary organic matter was oxidized to NH 4 + which was partially transformed to NO 2 −. NO 3 − production occurred only during spring when redox conditions were appropriate. Pore water NO 3 − profiles were greatly influenced by endofauna activity. These oxidation processes appeared slower than in the water column but organic and nutrient concentrations encountered were generally much higher. Molecular diffusion calculated from vertical pore water concentration gradients could not entirely explain fluxes observed throughout the year. The annual pattern of the exchanges of the remineralized compounds Si(OH) 4, NH 4 + , NO 3 − and NO 2 − at the sediment-water interface was studied under laboratory conditions using a core incubation method. These nutrient fluxes ranged over the year from − 24.8 to 47.7 μmol · m −2 · h −1 for ammonium, from − 15.6 to 94.1 μmol · m −2 · h −1 for nitrate, from − 10. to 9.3 μmol · m −2 · h −1 for nitrite and from −18.9 to 94.5 μmol · m −2 · h −1 for silicate. The annual variations of nitrate and nitrite fluxes have been found to be well correlated with the C : N ratio of the organic matter within the first centimetre of the sediment. Annual schedules of observed nutrient exchanges supplied to water column enrichment.