Distributions, trends and inter-annual variability of nutrients along a repeat section through the Weddell Sea (1996–2011)

• Unique series of repeat sections of the nutrients nitrate, phosphate and silicate across the Weddell Sea
• Nutrient data over a time span of 15 years with extraordinary consistent data due to standardization with the same nutrient reference water
• Extensive section and description of the distribution of nitrite in the Weddell Sea
• Strong increases of the nutrients in the surface layer due to increased rate of subsurface water upwelling in 15 years time span
• Silicate increase over 15 years in the bottom layer

Nutrient data from five repeat sections spanning 1996 to 2011 crossing the Weddell Sea are presented. These measurements have been standardized against the same reference material, yielding an outstanding internal consistency. The generic structure of the Weddell Gyre and its hydrographic features are visible in the nutrient distributions; variability is largest in the Circumpolar Deep Water (CDW) and only minor in other water masses. The distribution of silicate appears to be very powerful for describing water mass processes in the bottom layer. The distribution of nitrite is described in detail for the first time. Opposed to common knowledge, a considerable part of the abyssal Weddell Sea had detectable nitrite concentrations, hinting at biological activity at these depths. Also the bottom water, which definitely exists on a longer-than-seasonal scale, has a nitrite signature, thus challenging the commonly assumed short life-time of nitrite. We infer significant trends of increasing nutrient concentrations in the surface layer, which are attributed mainly to an increasing rate of upwelling of subsurface water over those 15 years. Also in the depth range of the CDW an increasing trend is found, which indicates that the nutrient supply to the gyre may have increased. In the bottom water, silicate exhibits an increasing trend, which is probably caused by a changing composition of the bottom water, i.e. containing more CDW and less surface water.