Bril Jeremy S, Langenfeld Kathryn, Just Craig L, Spak Scott N, Newton Teresa J
Department of Civil and Environmental Engineering, University of Iowa , Iowa City , IA , United States.
Upper Midwest Environmental Sciences Center, US Geological Survey , La Crosse , WI , United States.
PeerJ. 2017 Jan 4;5:e2838. doi: 10.7717/peerj.2838. eCollection 2017.
A freshwater "mussel mortality threshold" was explored as a function of porewater ammonium (NH) concentration, mussel biomass, and total nitrogen (N) utilizing a numerical model calibrated with data from mesocosms with and without mussels. A mortality threshold of 2 mg-N L porewater NH was selected based on a study that estimated 100% mortality of juvenile mussels exposed to 1.9 mg-N L NH in equilibrium with 0.18 mg-N L NH. At the highest simulated mussel biomass (560 g m) and the lowest simulated influent water "food" concentration (0.1 mg-N L), the porewater NH concentration after a 2,160 h timespan without mussels was 0.5 mg-N L compared to 2.25 mg-N L with mussels. Continuing these simulations while varying mussel biomass and N content yielded a mortality threshold contour that was essentially linear which contradicted the non-linear and non-monotonic relationship suggested by Strayer (2014). Our model suggests that mussels spatially focus nutrients from the overlying water to the sediments as evidenced by elevated porewater NH in mesocosms with mussels. However, our previous work and the model utilized here show elevated concentrations of nitrite and nitrate in overlying waters as an indirect consequence of mussel activity. Even when the simulated overlying water food availability was quite low, the mortality threshold was reached at a mussel biomass of about 480 g m. At a food concentration of 10 mg-N L, the mortality threshold was reached at a biomass of about 250 g m. Our model suggests the mortality threshold for juvenile species could be exceeded at low mussel biomass if exposed for even a short time to the highly elevated total N loadings endemic to the agricultural Midwest.
利用一个根据有无贻贝的中宇宙数据校准的数值模型，研究了淡水“贻贝死亡阈值”与孔隙水铵（NH）浓度、贻贝生物量和总氮（N）之间的函数关系。根据一项研究，选择孔隙水NH的死亡阈值为2 mg-N/L，该研究估计，暴露于与0.18 mg-N/L NH处于平衡状态的1.9 mg-N/L NH中的幼贻贝死亡率为100%。在模拟的最高贻贝生物量（560 g/m²）和最低模拟进水“食物”浓度（0.1 mg-N/L）下，在没有贻贝的2160小时时间段后，孔隙水NH浓度为0.5 mg-N/L，而有贻贝时为2.25 mg-N/L。在改变贻贝生物量和N含量的同时继续这些模拟，得到了一个基本呈线性的死亡阈值等值线，这与Strayer（2014）提出的非线性和非单调关系相矛盾。我们的模型表明，贻贝将上覆水中的营养物质空间集中到沉积物中，有贻贝的中宇宙中孔隙水NH升高就证明了这一点。然而，我们之前的工作以及这里使用的模型表明，贻贝活动的间接后果是上覆水中亚硝酸盐和硝酸盐浓度升高。即使模拟的上覆水食物可利用性相当低，在贻贝生物量约为480 g/m²时也会达到死亡阈值。在食物浓度为10 mg-N/L时，在生物量约为250 g/m²时达到死亡阈值。我们的模型表明，如果幼年贻贝短时间暴露于中西部农业地区特有的高总氮负荷下，即使贻贝生物量较低也可能超过其死亡阈值。
