Collaborative Research: RAPID: Nearshore settlement and hydrodynamics in Southern California during El Nino, and the transition to normal ocean conditions: boom and bust?

了解幼虫如何在沿海海洋中运输是表征海洋生物种群波动的关键。研究表明，生活在浅水区的物种的幼虫可以对不断变化的海洋条件做出行为反应，通过垂直移动到水流中，水流可以促进它们向沿海、近岸栖息地的运输，在那里它们定居在海底栖息地，完成它们的生命周期。然而，促进这种运输的海洋学机制，以及它们如何受到厄尔尼诺Niño等罕见事件的影响，目前还没有得到很好的解决。在气候变化的影响下，El Niño事件的频率和强度可能会增加，因此有必要评估El Niño对幼虫迁移和定居的影响。为此，本研究将使用一套前所未有的近岸生物和物理测量，跨越pre-El-Niño，在El Niño期间，以及预测返回El Niño中性条件，以机械方式测试幼虫在近岸海岸环境中的运输和定居反应。该项目还将为圣地亚哥大学（一所文理大学）的学生提供教育和研究机会。将为本科生开发至少一个实验室练习，展示El Niño对幼虫运输和定居的影响，并将招募学生参与项目的各个方面，为他们提供实践研究经验。本研究将构成至少一名硕士研究生论文工作的基础。最后，考虑到这项研究是在海洋保护区内进行的，研究结果将被广泛传播，并与海岸管理者和加州鱼类和野生动物部共享。幼虫的迁移和定居是了解底栖无脊椎动物种群动态的基本过程。先前的研究和未发表的观察结果表明，El Niño事件深刻影响了群落和种群过程，在南加州，El Niño的影响范围从改变幼虫的运输和当地种群的定居，到亚热带物种的地理扩张。本研究将验证当前（2015-2016）El Niño事件将导致南加州近岸栖息地藤壶幼虫运输和定居减少的假设。可能涉及两种机制；首先，厄尔尼诺Niño造成的温跃层加深会减少幼虫通过内部潮孔的运输，而这一机制需要温跃层变浅。其次，与El Niño中性条件相比，在El Niño期间，滨海藤壶的幼虫分布更深，更靠近近海，对近岸栖息地的限制更小，导致近岸幼虫丰度和定居减少。El Niño对南加州鸟岩近岸环流、水文、幼虫迁移和定居的影响将通过a)部署一系列测量温度、压力（波）和洋流的仪器来测量；B)测量每日藤壶幼虫沉降量；C)评估无脊椎动物幼虫的跨海岸和深度分布。这些观测结果将与2014年（El Niño中性条件）和2015年（El Niño期间）在Bird Rock进行的两年可比观测结果进行对比。此外，调查人员将每周测量Bird Rock和Dike Rock的沉降量，Dike Rock是一个向北7公里的地点，在1997/1998年El Niño结束时的先前观测表明，藤瓶沉降量非常高。这将能够评估El Niño条件下沉降响应的一般性，并检查沉降如何沿海岸线变化。

Understanding how larvae are transported in the coastal ocean is key for characterizing the population fluctuations of marine organisms. Studies demonstrate that larvae of species that inhabit shallow waters can behaviorally respond to changing oceanographic conditions by moving vertically into currents that can promote their transport to coastal, nearshore habitats where they settle to bottom habitats and complete their life cycle. However, the oceanographic mechanisms that promote such transport, and how they might be impacted by infrequent events such as El Niño, are poorly resolved. Given that El Niño events might increase in frequency and magnitude under climate change, it is imperative to assess how El Niño affects larval transport and larval settlement. To this end, this study will use an unprecedented set of nearshore biological and physical measurements spanning pre-El-Niño, during El Niño, and the predicted return to El Niño neutral conditions, to test mechanistically how larval transport and settlement respond in a nearshore coastal environment. This project will also provide educational and research opportunities for students at the University of San Diego, a liberal arts university. At least one laboratory exercise demonstrating the impacts of El Niño on larval transport and settlement will be developed for undergraduate students, and students will be recruited to participate in all aspects of the project to provide them with hands-on research experience. This research will form the basis for the thesis work of at least one M.S. graduate student. Finally, given that the research falls within a Marine Protected Area, results will be broadly disseminated and shared with coastal managers and the CA Department of Fish and Wildlife.Larval transport and settlement are fundamental processes for understanding the population dynamics of benthic invertebrates. Previous studies and unpublished observations indicate that El Niño events profoundly impact community and population processes, and in Southern California, El Niño effects range from alteration of larval transport and settlement of local populations, to the geographic expansion of subtropical species. This research will test the hypothesis that the current (2015-2016) El Niño event will result in a reduction of barnacle larval transport and settlement in Southern California nearshore habitats. Two mechanisms might be involved; first, a deepening of the thermocline forced by El Niño would result in reduction of larval transport by internal tidal bores, a mechanism that requires shallowing of the thermocline. Second, the distribution of larvae of littoral barnacles would be deeper, more offshore, and less constrained to nearshore habitats during El Niño than in El Niño neutral conditions, resulting in a reduction of nearshore larval abundance and settlement. The effects of El Niño on nearshore circulation, hydrography, larval transport and settlement in Bird Rock, Southern California, will be measured by a) deploying an array of instrumentation to measure temperature, pressure (waves) and currents; b) measuring daily barnacle larval settlement, and; c) assessing cross-shore and depth distribution of invertebrate larvae. These observations will be contrasted with two years of comparable observations taken at Bird Rock in 2014 (El Niño neutral conditions) and 2015 (during El Niño). Additionally, the investigators will measure weekly settlement at Bird Rock, and at Dike Rock, a site 7 km to the north, where previous observations at the end of the 1997/1998 El Niño indicated that barnacle settlement was very high. This will enable the evaluation of the generality of the settlement response as El Niño conditions eclipse, and examination of how settlement varies along a coastline.