Arctic terrestrial runoff has long been known to contribute to the coastal nutrient supply20,33, but understanding how Arctic greening may change the terrestrially–derived load of dissolved nutrients to the fjords and coastal systems remains unclear. Progress on this topic has been limited by a lack of interdisciplinary studies linking cross–ecosystem nitrate cycling, which we here aimed to overcome by combining, and by identifying the concurrent patterns in possible drivers of changing nitrate concentration in the surface waters.
The observations in this dataset spanning nearly two decades suggest intensified oligotrophication in the fjord system during summer (nitrate < 1 µM) and a 39% decrease in phytoplankton biomass. The rate of nitrate decline of 0.34 µM per decade for this area is lower than the western Arctic rate of decline of 1.1 µM per decade, yet about five times higher the estimated global rate (0.06 µM per decade)34. Overall, data presented here suggests that this Northeast Greenland fjord system has become progressively more oligotrophic over the past 20 years at a relatively high pace.
Several processes have apparently contributed to the observed decrease in nitrate concentration in the fjord system. First, the increasing freshening35–37 of the fjord surface water may result from increased freshwater runoff from the terrestrial catchment (i.e., Zackenberg River), or increased inflow of water from outside the fjord. Catchment runoff from the terrestrial areas showed only an 8% increase from 2003 to 2020, and there has been no increase in maximum snow depth9,30. Therefore, the observed freshening in the fjord is most likely due to the observed freshening of the water masses from outside the fjord36,38. The dilution by freshwater inflow of cold low–nitrate and low–saline water to the fjord system decreased surface water salinity by 4% over the two decades; direct dilution has probably reduced the surface water nitrate content to some extent. However, direct dilution would also result in a concomitant reduction in the concentrations of other macronutrients, but we observed no significant decline in phosphate concentration in the surface waters. Moreover, the change in the duration of the ice–free season also could not explain the decreasing trend, as the duration of the ice–free season did not show any significant change either. Therefore, inflow of nitrate–poor waters from the sea outside the fjord has most likely contributed to the reduction in the fjord nitrate content, but it cannot explain all of the observed nitrate reduction.
It could also be argued, however, that the decreasing trend in nitrate in the fjord was merely related to intensified stratification, but this is contradicted by the fact that there was no significant decreasing trend in the vertical flux of new nitrate from below the photic zone. The estimated nitrate flux of 0.23 mmol N m–2 d–1 was low compared to other areas37. These results suggest that productivity in the fjord system is limited by low nitrate availability caused by strong stratification and weak vertical mixing. This is consistent with the clear spatial gradients in the fjord, with decreasing productivity and Chl a biomass in the inner part of the fjords where the highest influence of run–off is found39,40. The decrease in productivity and Chl a biomass cannot be explained by a higher grazing by copepods, as there was no increase in zooplankton abundance 41.
Based on our synthesis of data from multiple ecosystems (i.e., terrestrial, fluvial, marine), we suggest that the decline in terrestrial nitrate export due to increased terrestrial greening is an important control of the reduction in nitrate availability in fjord systems. Although previous research has suggested that inorganic nutrient export per unit of stream discharge is expected to increase with climate warming in the Arctic20, our results suggest the opposite. In our study, the increase in greening of 20% NDVImax correlated with a decline in terrestrially–derived nitrate load to the fjord. The 20% increase in greening represents a 10% higher NDVImax than the increase in general Arctic tundra greenness observed between 1982–201910, indicating a substantial high rate of change in this high Arctic region. Together with the decline in the observed soil water nitrate, these results suggest a significant decline in terrestrial nitrate export to the fjord as the landscape is greening. The decline in terrestrial nitrate export correlated with a 39% decrease in phytoplankton biomass during our study period. Our results suggest that the decline in nitrate export from land contributed significantly to the surface water oligotrophication, resulting in a 49% reduction of surface water nitrate concentrations in this area (Fig. 3). The proposed link between terrestrial runoff and nutrient cycling in the fjord is further supported by the significant and positive correlation between riverine nitrate and fjord surface water nitrate.
Overall, our work strongly indicates that increasing terrestrial greening appears linked to the reduction in fjord nitrate concentrations and highlights the need to improve insights into the cross–boundary ecological consequences among ecosystems. Whether this scenario is specific only to this fjord, or can be generalized to other Arctic fjords, is still unknown and is an important focus area for future studies. In addition, as nitrogen and phosphorus are the primary limiting nutrients in the global ocean42,43, knowledge linking nutrient cycling processes and ecological consequences across ecosystems is crucial to understanding and modeling future changes in Arctic pelagic primary production. An important next step will be to further increase the mechanistic understanding on how present terrestrial and riverine characteristics affect the nitrogen export, which will support a clearer understanding about how the high–Arctic will respond to ongoing climatic changes.