Temporal variation and trends of inorganic nutrients in the coastal upwelling of the NW Spain (Atlantic Galician rías)

•Inorganic nutrients in Galician upwelling rías (NW Spain) were studied (2002–2011).•Surface nutrients showed a clear seasonal variability (max.: October–February).•The highest nutrients were recorded in the inner estuarine upwelling stations.•Series length, initial year and time integration period affect nutrient trends.•Ammonium and silicate trends were positive in the outer A0 st. (2002–2011, 1993–2011).

The temporal variability of inorganic nutrients (nitrate, nitrite, ammonium, phosphate and silicate) in five coastal upwelling Galician rías (Ría de Vigo, Ría de Pontevedra, Ría de Arousa, Ría de Muros and Ría de Ares-Betanzos) was assessed by considering biweekly values at 10 oceanographic stations for the decade 2002–2011. The long trends (1993–2011) of biweekly and annual average series for one station (mouth of Ría de Arousa, A0) were compared with decadal trends. A marked seasonal variability in all surface inorganic nutrients was observed. The average of the ten annual cycles (2002–2011) of each variable was calculated in order to obtain the average seasonal cycles. Maximum surface values occurred from the end of October to the end of February (8–24 μmol L− 1 dissolved inorganic nitrogen, 5–22 μmol L− 1 silicate and 0.5–1.8 μmol L− 1 phosphate). Minimum surface values (0.9–3.0 μmol L− 1 dissolved inorganic nitrogen, 0.6–2.5 μmol L− 1 silicate and 0.15–0.6 μmol L− 1 phosphate) detected from the beginning of May to the end of September. The highest nutrient concentrations (> 35%) recorded in the inner stations (V3, P3, A3, M2 and L3), as compared to the outer stations (V5, P4, A0, M5 and L1), indicated a more estuarine behaviour of these areas. Trends of the deseasonalised nutrient data were highly dependent on factors as length of the series, the initial year and the time integration period considered. The results found for these trends should be taken with caution due to the change in the sign of the slopes and the moderate accuracy of the models. Considering the variables whose trends did not change sign, ammonium and silicate showed a slightly positive trend for the biweekly-averaged series in A0 station during the periods 2002–2011 and 1993–2011.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slide