Nitrogen uptake rates and phytoplankton composition across contrasting North Atlantic Ocean coastal regimes north and south of Cape Hatteras

Abstract

Understanding nitrogen (N) uptake rates respect to nutrient availability and the biogeography of phytoplankton communities is crucial for untangling the complexities of marine ecosystems and the physical, biological, and chemical forces shaping them. In the summer of 2016, we conducted measurements of bulk microbial uptake rates for six ¹⁵N-labeled substrates: nitrate, nitrite, ammonium, urea, cyanate, and dissolve free amino acids across distinct marine provinces, including the continental shelf of the Mid-and South Atlantic Bights (MAB and SAB), the Slope Sea, and the Gulf Stream, marking the first instance of simultaneously measuring six different N uptake rates in this dynamic region. Total measured N uptake rates were lowest in the Gulf Stream followed by the SAB. Notably, the MAB exhibited significantly higher N uptake rates compared to the SAB, likely due to the excess levels of pre-existing phosphorus present in the MAB. Together, urea and nitrate uptake contributed approximately 50% of the total N uptake across the study region. Although cyanate uptake rates were consistently low, they accounted for up to 11% of the total measured N uptake at some Gulf Stream stations. Phytoplankton groups were identified based on specific pigment markers, revealing a dominance of diatoms in the shelf community, while Synechococcus, Prochlorococcus, and pico-eukaryotes dominated in oligotrophic Gulf Stream waters. The reported uptake rates in this study were mostly in agreement with previous studies conducted in coastal waters of the North Atlantic Ocean. This study suggests there are distinct regional patterns of N uptake in this physically dynamic region, correlating with nutrient availability and phytoplankton community composition. These findings contribute valuable insights into the intricate interplay of biological and chemical factors shaping N dynamics in disparate marine ecosystems.

Keywords: Cape Hatteras; Gulf Stream; Mid Atlantic Bight; South Atlantic Bight; cyanate; nitrogen uptake using 15N isotopes; phytoplankton community composition.

Figure 1

(A) Map of the Northwestern Atlantic Ocean showing the 8-day MODIS mean sea surface temperature (SST) from 4–11 August 2016. The white box indicates the study area. The overlaying black dotted and gray dashed lines are SSH contours of 0.2 and 0.5 m, respectively, in which the 0.2 m SSH contour denotes the Gulf Stream Edge (Muglia et al., 2022). (B) Zoom-in view of the MODIS SST image highlighting the Gulf Stream and colder Mid-Atlantic Bight (MAB) shelf water and warmer South Atlantic Bight (SAB) shelf water. The white circles represent 41 sampling stations occupied between August 7–17, 2016. (C) Bathymetry of the Northwestern Atlantic Ocean, highlighting the study area. (D) Zoomed-in view of the study area showing bathymetry across the southern MAB and northern SAB and the cruise transects, in which purple circles represent 27 stations where N uptake experiments using ¹⁵N tracers were performed. Also marked are 20, 50, 200, 1,000, 3,000, and 4,000 m isobaths in (D). The thick black solid line in all panels mark the 200 m isobaths. The black dotted line in panels (B) and (D) are SSH contours of 0.2 m.

Figure 2

Temperature (A–H), salinity (I–P), and density anomaly (Q–X) distributions along the eight cross-shelf transects 1–8. Light gray lines in transects 1–4 denote isohaline at 34.5 which delineate the MAB and Slope Sea waters. Gray dots indicate the depths where water samples were collected using Niskin bottles. Purple circles represent the depths at which N uptake incubations were conducted. Station numbers (1–41) are indicated at the top of each panel.

Figure 3

Temperature versus salinity (T − S) diagrams, superimposed with (A,B) chlorophyll a (Chl a) concentrations, (C,D) depth, and (E,F) color coded water masses, based on the 1 db binned CTD data collected during the August 2016 cruise. Gray contours represent isohalines for density anomaly (sigma, kg m⁻³). The right panels are the same as those on the left but are zoomed-in to focus on the salinity range from 35.5 and 37. GS, SAB, Upw, Front, SS, and MAB indicate waters from the Gulf Stream, South Atlantic Bight, upwelling, Slope Sea–Gulf Stream front, Slope Sea, and Mid-Atlantic Bight, respectively. DSIW is deep slope water (Flagg et al., 2002). The major water masses in the sampling area include MAB shelf water (S < 34.5) and Slope Sea water (34.5 < S < 36), Slope Sea-Gulf Stream frontal water (36 < S < 36.5), SAB shelf water (36 < S < 36.5), and Gulf Stream (S > 36.5). Note that in panels (E,F), color coded water masses at the surface did not strictly follow the above salinity criteria.

Figure 4

Chlorophyll (Chl, A–H), nitrate (NO₃⁻, I–P), and phosphate (PO₄, Q–X) concentrations along the eight cross-shelf transects 1–8. Light gray lines in transects 1–4 denote the 34.5 isohaline which delineates the MAB water and Slope Sea waters in transects north of Cape Hatteras. Gray dots indicate the depths where water samples were collected using Niskin bottles. Purple circles represent the depths at which N uptake experiments were conducted. Station numbers (1–41) are indicated at the top of each panel.

Figure 5

Depth profiles of (A) nitrate ([NO₃⁻]) and (B) PO₄ ([PO₄]) concentrations from the MAB (dark purple circles) and SAB (orange diamonds) as well as (C) property vs. property plot of [NO₃⁻]and [PO₄], superimposed with P* contours spanning from −0.2 to +0.6 incremented by 0.1, where P* = [PO₄] – [NO₃⁻]/16.

Figure 6

Nitrite (NO₂⁻, A–H), ammonium (NH₄⁺, log transformed, I–P), and cyanate (Q–X) concentration distributions along the eight cross-shelf transects 1–8. Light gray lines in transects 1–4 denote isohaline at 34.5 which delimit the MAB water and Slope Sea. Gray dots indicate the depths where water samples were collected using Niskin bottles. Purple circles represent the depths at which N uptake incubations were conducted. Station numbers (1–41) are indicated at the top of each panel.

Figure 7

Vertical distribution of whole community N uptake rates including nitrate (NO₃⁻, triangles), nitrite (NO₂⁻, crosses), ammonium (NH₄⁺, diamonds), urea (squares), dissolved free amino acids (DFAA, asterisks), and cyanate (circles) in (A) Mid-Atlantic Bight (MAB), (B) Slope Sea, (C) South Atlantic Bight (SAB), and (D) Gulf Stream waters. The depth range of y axis at each panel is different because depths of Chl maximum deepened from onshore to offshore.

Figure 8

Plots of (A–C) stacked N uptake rates, (D–F) the fraction of total N uptake contributed by the various N species measured, and (G–I) corresponding particulate nitrogen (PN, black dots) and Chl a (green dots) concentrations across different regions which delimited by gray lines. MAB, SS, SAB stand for Mid Atlantic Bight, Slope Sea, and South Atlantic Bight, respectively. The left, middle, and right panels represent data from surface, above the Chl maximum, and Chl maximum depths, respectively. The upper two panels share the same color palette, and each color represents one of the six tested N species. DFAA in the color legend stands for dissolved free amino acids. Black dots in the upper panel indicate that incubation was conducted at nighttime.

Figure 9

Spatial distribution of N uptake rates contributed by the six different N species, superimposed with (A–C) in-situ temperature and (D–F) PO₄ concentrations ([PO₄]) measured at the surface (upper panels), above the Chl maximum (middle panels), and at the Chl maximum depth (bottom panels). The size of each pie in the figure represents the magnitude of the total N uptake, and each colored slice corresponds to one of the six tested N species. The color legend is provided in panel (F). Contrasts in temperature and PO₄ concentrations among the MAB, SAB, SS, and GS regions are observed. The thick black solid line in all panels mark the 200-m isobaths. The black dotted lines in upper panels represent the Gulf Stream Edge.

Figure 10

Same as Figure 7, but for specific N uptake rates, which are independent from particulate nitrogen concentrations.

Figure 11

Phytoplankton community composition at the surface water (A,D, left panels), above the Chl maximum (B,E, middle panels), and Chl maximum depths (C,F, right panels) in four distinct regimes separated by the gray solid lines, including Mid-Atlantic Bight (MAB), Slope Sea (SS), South Atlantic Bight (SAB), and Gulf Stream. The total stacked bar length in upper panels represent the total Chl a concentrations, and the composition graph in lower panels reflects the relative contributions of different phytoplankton groups to total Chl a determined from CHEMTAX assessment of diagnostic pigments.

Figure 12

Heatmap of correlation coefficients among N uptake rates and environmental factors including temperature, salinity, ambient nutrient concentrations ([PO₄], [NO₃⁻], [NO₂⁻], [NH₄⁺], [DFAA], [Cyanate]), chlorophyll concentrations, particulate nitrogen [PN], and phytoplankton species (in %). Total N_R, NO₃_R, NO₂_R, NH₄_R, urea_R, AAs_R, Cyanate_R stands for the uptake rate of total nitrogen, nitrate, nitrite, ammonium, urea, amino acids, and cyanate, respectively. DFAA stands for dissolved free amino acids.

Figure 13

Redundancy analysis (RDA) plot showing the correlations between response variables (N uptake rates, indicated by orange arrows) and a set of explanatory variables (lavender vectors), including temperature (Temp), salinity, ambient nutrient concentrations ([phosphate], [nitrate], [nitrite], [ammonium], and [DFAA]), Chl and PN concentrations ([Chl], [PN]), and phytoplankton community composition including diatoms (Diatom), dinoflagellates (Dino), haptophytes [Type 8; Hapt(T8)], haptophytes [Type 6; Hapt(T6)], chlorophytes (Chlo), cryptophytes (Cryp), Prochlorococcus (Proc), Synechococcus (Syne), and prasinophytes (Pras). The dark purple dots indicate observations (a total of 56 after excluding data with NaNs). Note that urea was not included due to undetectable concentrations, and ammonium was not shown in the plot due to its small vector size. Total uptake, NO₃_R, NO₂_R, NH₄_R, Urea_R, DFAA_R, Cyanate_R stands for the uptake rate of total nitrogen, nitrate, nitrite, ammonium, urea, amino acids, and cyanate, respectively. DFAA stands for dissolved free amino acids.