Sundby Svein, Kristiansen Trond
Institute of Marine Research and Hjort Centre for Marine Ecosystem Dynamics, P.O. Box 1870 Nordnes, 5817, Bergen, Norway.
PLoS One. 2015 Oct 14;10(10):e0138821. doi: 10.1371/journal.pone.0138821. eCollection 2015.
Buoyancy acting on plankton, i.e. the difference in specific gravity between plankton and the ambient water, is a function of salinity and temperature. From specific gravity measurements of marine fish eggs salinity appears to be the only determinant of the buoyancy indicating that the thermal expansions of the fish egg and the ambient seawater are equal. We analyze the mechanisms behind thermal expansion in fish eggs in order to determine to what extent it can be justified to neglect the effects of temperature on buoyancy. Our results confirm the earlier assumptions that salinity is the basic determinant on buoyancy in marine fish eggs that, in turn, influence the vertical distributions and, consequently, the dispersal of fish eggs from the spawning areas. Fish populations have adapted accordingly by producing egg specific gravities that tune the egg buoyancy to create specific vertical distributions for each local population. A wide variety of buoyancy adaptations are found among fish populations. The ambient physical conditions at the spawning sites form a basic constraint for adaptation. In coastal regions where salinity increases with depth, and where the major fraction of the fish stocks spawns, pelagic and mesopelagic egg distributions dominate. However, in the larger part of worlds' oceans salinity decreases with depth resulting in different egg distributions. Here, the principles of vertical distributions of fish eggs in the world oceans are presented in an overarching framework presenting the basic differences between regions, mainly coastal, where salinity increases with depth and the major part of the world oceans where salinity decreases with depth. We show that under these latter conditions, steady-state vertical distribution of mesopelagic fish eggs cannot exist as it does in most coastal regions. In fact, a critical spawning depth must exist where spawning below this depth threshold results in eggs sinking out of the water column and become lost for recruitment to the population. An example of adaptation to such conditions is Cape hake spawning above the critical layer in the Northern Benguela upwelling ecosystem. The eggs rise slowly in the onshore subsurface current below the Ekman layer, hence being advected inshore where the hatched larvae concentrate with optimal feeding conditions.
作用于浮游生物的浮力，即浮游生物与周围水体之间的比重差异，是盐度和温度的函数。根据对海鱼卵比重的测量，盐度似乎是浮力的唯一决定因素，这表明鱼卵和周围海水的热膨胀是相等的。我们分析了鱼卵热膨胀背后的机制，以确定在何种程度上可以合理地忽略温度对浮力的影响。我们的结果证实了早期的假设，即盐度是海鱼卵浮力的基本决定因素，进而影响鱼卵的垂直分布，并因此影响鱼卵从产卵区的扩散。鱼类种群通过产生特定比重的鱼卵来进行相应的适应，从而调整鱼卵的浮力，为每个当地种群创造特定的垂直分布。在鱼类种群中发现了各种各样的浮力适应方式。产卵地的周围物理条件构成了适应的基本限制。在盐度随深度增加的沿海地区，也是大部分鱼类种群产卵的地方，浮游和中层鱼卵分布占主导。然而，在世界上大部分海洋中，盐度随深度降低，导致鱼卵分布不同。在这里，世界海洋中鱼卵垂直分布的原理在一个总体框架中呈现，展示了不同区域之间的基本差异，主要是沿海地区，那里盐度随深度增加，以及世界海洋的大部分地区，那里盐度随深度降低。我们表明，在后者这些条件下，中层鱼卵的稳态垂直分布无法像在大多数沿海地区那样存在。事实上，必须存在一个临界产卵深度，在这个深度阈值以下产卵会导致鱼卵沉入水柱之外，从而无法补充到种群中。适应这种条件的一个例子是北大西洋本格拉上升流生态系统中，无须鳕在临界层以上产卵。鱼卵在埃克曼层以下的近岸次表层水流中缓慢上升，因此被平流到近岸，孵化出的幼鱼在那里集中，具备最佳的摄食条件。
