Groundwater sources of "new" N for benthic microalgal production in the South Atlantic Bight

大陆架具有很高的生产力，在生态和经济上都很重要。底栖微藻（BMA）是这些地区的主要初级生产者。多达6倍的初级生产者水柱生物量被压缩到沉积物表面只有几毫米厚的一层。支持这种高浓度BMA生物质的固定氮(N)来源目前尚不清楚。南卡罗来纳大学最近对海底地下水流动的研究表明，上涌的含盐地下水可能为南大西洋湾（SAB）的脊滩栖息地提供高浓度的营养物质。研究人员认为，地下水向表层沉积物中输入的固定氮是支持南大陆架中部陆架地区BMA生物量和产量的主要氮来源。该项目的目的是确定浅海陆架水域（30米）表层沉积物中支持BMA生物量的固定N的主要来源，将SAB作为实地区域。第二个目标是应用新颖和创新的方法来直接量化地下水对地表沉积物的氮输入。研究结果将全面记录南海带中陆架地区BMA和浮游植物生物量的时空分布和群落结构，并将观测到的格局与固定氮源地下水输入以及水文和气候条件联系起来。本研究将为2名研究生提供全额支持和学费，为本科生助理提供暑期支持，并邀请高年级本科生作为实验室实习生。研究小组还将与巴鲁克研究所和其他合作伙伴合作，制定一个“海洋校园”项目，以满足教师、学生和社区观众的需求。该项目还将为Girls Go for it提供部分支持，这是一个旨在吸引中学女生从事it和STEM领域职业的编程夏令营。该研究的具体目标是(1)量化与水动力交换和地下水向上流动相关的氮通量的时空变化；(2)记录BMA生物量的时空变化；(3)测量固定氮源(井水、孔隙水和水柱铵硝态氮；沉积物)、BMA和浮游植物。采样区域将被限制在南卡罗来纳州查尔斯顿海岸外SAB的10 - 30米等深区域。样本将在现有的地下水井场和大陆架的其他区域收集。在井场的每口地下水井，潜水员将从井中收集液体，以确定井水中无机营养物质的浓度（硝酸盐+亚硝酸盐、铵、正磷酸盐、硅）和井水中铵和硝酸盐的δ 15N（当存在时）。在附近的沉积物中，将采集BMA生物量和群落组成、表面孔隙水无机营养物（硝酸盐+亚硝酸盐、铵、正磷酸盐、硅）、沉积物的C和N、沉积物粒度分析以及BMA、铵、硝酸盐（如果存在）和沉积物的δ 15N。将在4个深度地层中分别沿50米样带进行5个采样点的线样带。每隔10米，潜水员将在井场收集与上述相同的样品。将收集水柱样品，用于HPLC测定浮游植物生物量和群落组成、无机营养物浓度（硝酸盐+亚硝酸盐、铵、正磷酸盐、硅）、浮游植物CHN、浮游植物δ 15N、铵和硝酸盐δ 15N。研究人员将使用热量作为示踪剂来绘制水动力交换的深度，并监测垂直地下水流动的速率。该分析的结果还将允许他们模拟保守示踪剂的运输，该示踪剂可以与观察到的营养浓度进行比较，以获得BMA丰度和群落组成。

Continental shelves are highly productive, with both ecological and economic importance. Benthic microalgae (BMA) are key primary producers in these location. As much as 6x the water column biomass of primary producers is compressed into a layer only a few mm thick on the sediment surface. The source(s) of fixed nitrogen (N) supporting such highly concentrated BMA biomass is currently unknown. Recent studies of sub-seafloor groundwater flow at the University of South Carolina have demonstrated that upwelling saline groundwater likely supplies high concentrations of nutrients in the ridge-swale habitats in the South Atlantic Bight (SAB). The investigators suggest that groundwater input of fixed N into surficial sediments is the primary source of N supporting BMA biomass and production in the mid-shelf region of the SAB. The purpose of this project is to determine the primary source of fixed N supporting BMA biomass in the surface sediments of the shallow shelf waters (30 m), using the SAB as a field area. A secondary objective is to apply novel and innovative methods to directly quantify groundwater inputs of N into surficial sediments. Research results will fully document the spatio-temporal distributions of BMA and phytoplankton biomass and community structure in the mid-shelf region of the SAB and relate the observed patterns to groundwater inputs of fixed N sources as well as hydrographic and climatic conditions. This research will provide full support and tuition for 2 graduate students, summer support for undergraduate assistants, and involve upper level undergraduates as lab interns. The study team will also work with the Baruch Institute and other partners to develop an "Ocean Schoolyard" program to meet the needs of teachers, students, and community audiences. The project will also provide partial support for Girls Go for I.T., a coding summer camp designed to attract middle-school-aged girls to careers in I.T. and STEM fieldsThe specific objectives of the study are to (1) quantify spatial and temporal variations in N fluxes associated with hydrodynamic exchange and upward groundwater flow (2) document spatial and temporal variations in BMA biomass and (3) measure the delta15N of fixed nitrogen sources (well water, porewater and water column ammonium and nitrate; sediments), the BMA, and phytoplankton. The sampling area will be restricted to the 10 - 30 m isobath region of the SAB off the coast of Charleston, SC. Samples will be collected at both the existing groundwater well field and other regions of the shelf. At each of the groundwater wells in the well field, SCUBA divers will collect fluids from the wells to determine well water inorganic nutrient concentrations (nitrate + nitrite, ammonium, orthophosphate, silicon) and the delta 15N of well water ammonium and nitrate (when present). In nearby sediments, samples will be collected for BMA biomass and community composition, surface porewater inorganic nutrients (nitrate + nitrite, ammonium, orthophosphate, silicon), C and N of sediments, sediment grain size analysis, and delta 15N of BMA, ammonium, nitrate (when present), and sediments. Line transects, consisting of 5 sampling locations along a 50 m transect, will be conducted in each of the 4 depth strata. At 10 m intervals along each transect, divers will collect samples the same as above for the well field. Water column samples will be collected for HPLC measurements of phytoplankton biomass and community composition, inorganic nutrient concentrations (nitrate + nitrite, ammonium, orthophosphate, silicon), seston CHN, delta 15N of phytoplankton, and the delta 15N of ammonium and nitrate. The researchers will use heat as a tracer to map the depth of hydrodynamic exchange and monitor the rate of vertical groundwater flow. Results from that analysis will also allow them to then simulate transport of a conservative tracer that can be compared to observed nutrient concentrations to BMA abundance and community composition.