Abstract
Riverine nutrients acting in concert with local hydrographic conditions create distinct ecological niches for phytoplankton communities across river-ocean continuums. Here we compare two of the world’s largest river-ocean systems, the Amazon River Plume (ARP) which outflows into the Western Tropical North Atlantic (WTNA) and the Changjiang Diluted Water (CDW) which empties into the East China Sea to show how distinctly different N: P ratios of their source waters, shape phytoplankton communities along the river-ocean continuum. Sampling in the relatively unpolluted surface waters of the ARP during peak river discharge revealed that phytoplankton communities along the river-ocean continuum were strongly controlled by Dissolved Inorganic Nitrogen (DIN, nitrate plus nitrite) concentrations which were low or beyond detectable, while Dissolved Inorganic Phosphorous (DIP, phosphate) was not limiting. Downstream of the plume, nutrient concentrations at the surface were far below the Redfield N:P ratio of 16 : 1, and in this severely nitrogen limited but silica replete mesohaline region, Diatom-Diazotroph Associations (DDA) such as Hemiaulus hauckii-Richelia proliferated, while offshore in nitrate poor oceanic waters, Trichodesmium spp. and Prochlorococcus thrived. In contrast, in the CDW, a system which has witnessed a substantial rise in anthropogenic nitrogen inputs from human pressures within the Changjiang River system, phytoplankton community structure was regulated primarily by DIP which was rapidly consumed in the inner shelf. Data from a series of summer-time cruises in the mid-shelf and offshore regions of the East China Sea, at peak discharge, established that when DIP limitation was ameliorated by on-shelf upwelling of DIP rich Kuroshio Intermediate Waters, diatoms and dinoflagellates dominated. Conversely, during years of heavy discharge, the westward flowing CDW plume was severely DIP limited, probably because water column stratification dampened upwelling of subsurface waters. The consequent phosphate limitation led to the proliferation of small phytoplankton such as Chlorophytes and Cyanobacteria. The absence of diazotrophs in the CDW, leads us to hypothesize that river-ocean continuums, whose source waters are heavily impacted by anthropogenic activities and with nutrient concentrations substantially in excess of Redfield ratios, may not support diatoms on account of DIP limitation nor DDAs and diazotrophs because of excess DIN.
Highlights
Rivers are the primary conduit transporting weathered, leached, and human-derived material from land to the oceans (Sharples et al, 2017)
Low Excess Nitrate (ExN) arises from a combination of rapid uptake of Dissolved Inorganic Nitrogen (DIN) in the plume and the addition of Dissolved Inorganic Phosphate (DIP) from leaching (Fox et al, 1986)
It is likely that the N:P supply ratios from the two systems determine whether or not diazotrophs are part of the community, while it is likely that supply of a single limiting nutrient is responsible for the interannual variability seen in the East China Sea (ECS)
Summary
Rivers are the primary conduit transporting weathered, leached, and human-derived material from land to the oceans (Sharples et al, 2017). Several studies show that anthropogenic disturbance of river nutrient loads and export to coastal and ocean marine systems has increased, creating a global problem affecting water quality, and biodiversity (Bouwman et al, 2005). Increasing inputs of nitrogen and phosphorous from human activity, predominantly from land-based activities, have the potential to modify oceanic, and even global, biogeochemical systems. The total river input of nitrogen and phosphorous to the coastal seas has approximately doubled over the last few hundred years (Seitzinger et al, 2010; Beusen et al, 2016; Jickells et al, 2017). Sharples et al (2017) calculated that globally 75 and 80% Dissolved Inorganic Nitrogen (DIN) and Dissolved Inorganic Phosphate (DIP) respectively, reaches the open ocean these are considered as the upper limits as estuarine processes were ignored in the study. Low latitude rivers such as the Amazon, and the rivers in Southeast Asia including the Changjiang River are major sources of nutrients for the oceans or seas into which they discharge
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