Transitional areas between ecosystem types are often active biogeochemically due to resource limitation changes. Lotic-to-lentic transitions in freshwaters appear active biogeochemically, but few studies have directly measured nutrient processing rates to assess whether processing within the rivermouth is important for load estimates or the local communities. We measured oxic fluxes of inorganic nitrogen and phosphorus and dissolved organic carbon (DOC) from sediments in two rivermouths of Green Bay (Lake Michigan, USA). Soluble reactive phosphorus (SRP) flux was positive in most cases (overall mean 1.74 mg SRP m − 2 day − 1 ), as was ammonium (NH 4 ) flux (40.6 mg NH 4 m − 2 day − 1 ). Partial least square regression (PLSR) indicated a latent variable associated with both sediment [loosely bound phosphorus (P), iron bound P, organic content] and water column properties [temperature, DOC:dissolved inorganic nitrogen (DIN) and DOC:SRP ratios (negatively)] that was moderately associated with variation in SRP flux. PLSR analysis also indicated several sediment characteristics were moderately related to NH 4 flux, especially organic content, density (negative), and porosity. Flux of nitrates/nitrites (NO X ) and DOC were positively associated with the water column concentrations of NO X and DOC and qualitative estimates of the labile, non-humic types of DOC. In early summer, water column NO X and DOC concentrations were high and labile DOC may have fueled denitrification, resulting in net flux into sediments of both NO X and DOC. By late summer, water column NO X and DOC were very low and both these constituents were fluxing out of sediments into the water column. Based on our estimates for the entire period from April through September, rivermouth sediments were a net source of SRP and DIN, with a DIN:SRP ratio of ~ 44 and a NH 4 :NO X > 1. We estimated that the sediments in the Fox rivermouth probably contributed a small proportion of the total Fox River load during the growing season 2016 (< 5%), but at times may have contributed as much as 14% of the daily load. Despite the small size of the Fox rivermouth (< 0.5% of the watershed area), these results indicate that at times sediments can contribute substantially to the overall delivery of nitrogen and phosphorus to the nearshore zone.

中文翻译：

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进入格林湾的淡水河口沉积物中的磷、氮和溶解有机碳通量（密歇根湖；美国）

由于资源限制的变化，生态系统类型之间的过渡区通常在生物地球化学上很活跃。淡水中 Lotic 到 lentic 的转变在生物地球化学上显得活跃，但很少有研究直接测量养分加工速率来评估河口内的加工是否对负荷估计或当地社区很重要。我们测量了绿湾（美国密歇根湖）两个河口沉积物中无机氮、磷和溶解有机碳 (DOC) 的有氧通量。在大多数情况下，可溶性活性磷 (SRP) 通量为正（总体平均 1.74 mg SRP m - 2 day - 1 ），铵（NH 4 ）通量（40.6 mg NH 4 m - 2 day - 1 ）也是如此。偏最小二乘回归 (PLSR) 表明与沉积物 [松散结合磷 (P)、铁结合 P、有机物含量]和水柱特性 [温度、DOC:溶解无机氮 (DIN) 和 DOC:SRP 比率（负值）] 与 SRP 通量的变化适度相关。PLSR 分析还表明，几种沉积物特征与 NH 4 通量中度相关，尤其是有机物含量、密度（负）和孔隙度。硝酸盐/亚硝酸盐 (NO X ) 和 DOC 的通量与 NO X 和 DOC 的水柱浓度以及不稳定的非腐殖型 DOC 的定性估计呈正相关。在初夏，水体中 NO X 和 DOC 的浓度很高，不稳定的 DOC 可能促进了反硝化作用，导致 NO X 和 DOC 进入沉积物的净通量。到夏末，水柱中的 NO X 和 DOC 非常低，这两种成分都从沉积物中流入水柱。根据我们对 4 月至 9 月整个时期的估计，河口沉积物是 SRP 和 DIN 的净来源，DIN:SRP 比率约为 44，NH 4 :NO X > 1。我们估计Fox Rivermouth 可能在 2016 年生长季节贡献了 Fox River 总负荷的一小部分（< 5%），但有时可能贡献了高达 14% 的每日负荷。尽管 Fox 河口的面积很小（< 流域面积的 0.5%），但这些结果表明，有时沉积物可以对向近岸区域输送氮和磷的总体贡献很大。我们估计 Fox 河口的沉积物在 2016 年生长季节可能占 Fox River 总负荷的一小部分（< 5%），但有时可能占每日负荷的 14%。尽管 Fox 河口的面积很小（< 流域面积的 0.5%），但这些结果表明，有时沉积物对向近岸区域输送氮和磷的总体贡献很大。我们估计 Fox 河口的沉积物在 2016 年生长季节可能占 Fox River 总负荷的一小部分（< 5%），但有时可能占每日负荷的 14%。尽管 Fox 河口的面积很小（< 流域面积的 0.5%），但这些结果表明，有时沉积物可以对向近岸区域输送氮和磷的总体贡献很大。