Sediment nutrient fluxes, denitrification rates and sediment characteristics in Long Island Sound, NY

We collected sediment cores at five stations in Long Island Sound along a west to east transect to capture a range of environmental conditions within the estuary. We measured sediment fluxes of dissolved inorganic nutrients and dissolved gases at Execution Rocks (EXR), Western Long Island Sound (WLIS), the ARTG Buoy (ARTG), Central Long Island Sound (CLIS) and Eastern Long Island Sound (ELIS). Stations were sampled twice during the summer (July 2016, June 2017, August 2017) and once during the winter (December 2016).

In the field, we measured bottom dissolved oxygen (O₂), salinity, and temperature (Hach HQd using LD101, CDC401 probes) at each station. We collected duplicate water samples for quantification of dissolved inorganic nutrient concentrations (NH₄⁺, NO₃^-, NO₂^-, DIP). Nutrient samples were run on a Seal Auto Analyzer 3 with segmented flow injection using standard colorimetric techniques (Solorzano 1969; Johnson and Petty 1983; Grasshoff et al. 2009). We also collected surface water samples for chlorophyll-a analysis (Foster and Fulweiler 2014).

Sediment cores were collected using an Ocean Instruments MC-400 multi-corer. Cores were incubated and sampled for dissolved O₂ and dissolved inorganic nutrients (NH₄⁺, NO₃^-, NO₂^-, DIP) at the start and end of each incubation. During the incubation we collected samples for N₂ and Ar concentrations and N₂O and CH₄ concentrations (Fulweiler and Nixon 2009; Foster and Fulweiler 2014). N₂ and Ar concentrations were determined using using a quadrupole membrane inlet mass spectrometer (MIMS, Bay Instruments). N₂O and CH₄ concentrations were determined using a gas chromatograph (GC-2014, Shimadzu, Japan).

At the end of each incubation sediment samples were collected to determine sediment density, porosity, percent carbon (%C), percent nitrogen (%N), molar carbon to nitrogen (C:N) ratios and sediment chlorophyll-a. To determine sediment porosity, density we used techniques described by Nielsen et al. (2000). Methods described by Zimmermann et al. (1997) were used to determine %C %N. Percent C and N samples were run on an elemental analyzer at the Fichot Lab at Boston University. Sediment chlorophyll-a concentrations were determined using methods described by (Foster and Fulweiler 2014).

Email questions and comments to cmazur@bu.edu

EXR = Execution Rocks (40° 53.00' N 73° 43.70' W, 18 m deep)

WLIS = Western Long Island Sound (40° 57.35' N 73° 34.80' W, 19.8 m deep)

ARTG = ARTG Buoy (41° 00.60' N 73° 17.29' W, 29 m deep)

CLIS = Central Long Island Sound (41° 08.25' N 72° 39.30' W, 26.8 m deep)

ELIS = Eastern Long Island Sound (41° 15.80' N, 72° 04.00' W, 22.9 m deep)

Date = dd (day) - month - yy (year)

Distance = Distance from NYC, meters

Temp = Temperature, °C

WC_Chla = Surface water chlorophyll-a, µg L^-1

Density = g/mL

Porosity = sediment pore space to total volume ratio

C = percent carbon of total mass

N = percent nitrogen of total mass

CN = carbon to nitrogen molar ratio

Sed_Chla = sediment chlorophyll-a, mg m^-2

Sed_Phaeo = Sediment phaeophytin, mg m^-2

Bottom_DO = Bottom water dissolved oxygen, µmol L^-1

Bottom_NH4 = Bottom water ammonium, µmol L^-1

Bottom_NO2 = Bottom water nitrite, µmol L^-1

Bottom_NO3 = Bottom water nitrate, µmol L^-1

Bottom_NOx = Bottom water nitrite + nitrate, µmol L^-1

Bottom_DIN = Bottom water nitrate + nitrite + ammonium, µmol L^-1

Bottom_DIP = Bottom water dissolved inorganic phosphorus, µmol L^-1

Initial_NH4 = ammonium concentration at the start of incubations, µmol L^-1

Initial_NO2 = nitrite concentration at the start of incubations, µmol L^-1

Initial_NO3 = nitrate concentration at the start of incubations, µmol L^-1

Initial_NOx = nitrite + nitrate concentration at the start of incubations, µmol L^-1

Initial_DIN = nitrite + nitrate + ammonium concentration at the start of incubations, µmol L^-1

Initial_DIP = dissolved inorganic phosphorus concentration at the start of incubations, µmol L^-1

N2N = Net denitrification rate, µmol N₂-N m^-2 hr^-1

SOD = Sediment oxygen demand, µmol O₂ m^-2 hr^-1

CH4 = methane flux, nmol CH₄ m^-2 hr^-1

N2O = nitrous oxide flux, nmol N₂O m^-2 hr^-1

NH4 = ammonium flux, µmol NH₄⁺ m^-2 hr^-1

NO2 = nitrite flux, µmol NO₂^- m^-2 hr^-1

NO3 = nitrate flux, µmol NO₃^- m^-2 hr^-1

NOx = nitrite + nitrate, µmol NO_x m^-2 hr^-1

DIN = ammonium + nitrite + nitrate flux, µmol N m^-2 hr^-1

DIP = dissolved inorganic phosphorus flux, µmol DIP m^-2 hr^-1

DIN.DIP = nitrite + nitrate + ammonium to dissolved inorganic phosphorus ratio

We thank Aidan Borkan, Craig Tobias, Sydney Twarz, and the University of Connecticut Department of Marine Sciences Vessel Operations for their help with sediment core preparation and collection. We also thank Qile Chen and Carly Langan for helping process sediment samples. Thanks to Nilotpal Ghosh for help with analysis of sediment percent carbon and nitrogen and to Cédric Fichot for allowing us to use his elemental analyzer.

Foster SQ and RW Fulweiler. 2014. Spatial and historic variability of benthic nitrogen cycling in an anthropogenically impacted estuary. Frontiers in Marine Science 1. https://doi.org/10.3389/fmars.2014.00056.

Fulweiler RW, Nixon SW. 2009. Responses of benthic-pelagic coupling to climate change in a temperate estuary. Hydrobiologia 629:147–156. https://doi.org/10.1007/s10750-009-9766-0

Grasshoff K, Kremling K, Ehrhardt M. 2009. Methods of seawater analysis, 3rd edn. Wiley‐VCH, Verlag GmbH, D‐69469, Weinheim, Germany.

Johnson KS, Petty RL.1983. Determination of nitrate and nitrite in seawater by flow injection analysis. Am Soc Limnol Oceanogr 28:1260–1266.