﻿486 
  REPORT 
  OF 
  COMMISSIONER 
  OF 
  FISH 
  AND 
  FISHERIES. 
  

  

  forms 
  there 
  present 
  furnish 
  a 
  plenteous 
  food 
  supplj^ 
  for 
  the 
  3"oung 
  fish. 
  

   Although 
  the 
  larger 
  plants 
  as 
  such 
  are, 
  while 
  living, 
  little 
  used 
  as 
  food 
  

   by 
  the 
  aquatic 
  animals, 
  yet 
  they 
  greatly 
  increase 
  the 
  surface 
  available 
  

   for 
  the 
  attachment 
  of 
  microscopic 
  plant 
  forms, 
  which 
  are 
  eaten 
  by 
  the 
  

   smaller 
  animals, 
  and 
  the 
  latter 
  in 
  their 
  turn 
  by 
  the 
  fishes. 
  This 
  relation 
  

   of 
  the 
  larger 
  plants 
  to 
  the 
  food 
  supply 
  is, 
  as 
  Seligo 
  (1890, 
  pp. 
  46, 
  47) 
  

   pointed 
  out, 
  chiefly 
  mechanical 
  and 
  indirect. 
  

  

  The 
  second 
  role 
  usually 
  assigned 
  to 
  water 
  plants 
  is 
  that 
  of 
  aeration, 
  

   in 
  which 
  the 
  plants 
  by 
  their 
  carbon 
  assimilation 
  remove 
  carbon 
  dioxid 
  

   from 
  the 
  water 
  and 
  give 
  out 
  oxygen 
  in 
  its 
  place. 
  Aquatic 
  animals 
  use 
  

   the 
  oxygen 
  which 
  is 
  in 
  solution 
  in 
  the 
  water 
  and 
  give 
  off 
  carbon 
  dioxid, 
  

   which 
  passes 
  into 
  the 
  water, 
  and 
  which, 
  if 
  it 
  should 
  accumulate 
  exces- 
  

   sively, 
  would 
  ])ecome 
  fatal 
  to 
  the 
  animals. 
  The 
  water 
  must, 
  then, 
  be 
  

   constantly 
  supplied 
  with 
  fresh 
  oxygen 
  and 
  as 
  constantly 
  freed 
  of 
  the 
  

   greater 
  part 
  of 
  its 
  carbon 
  dioxid. 
  In 
  sunlight 
  plants 
  absorb 
  carbon 
  

   dioxid, 
  and 
  in 
  using 
  it 
  for 
  the 
  manufacture 
  of 
  carbon 
  compounds 
  give 
  

   off 
  oxygen 
  to 
  the 
  water 
  in 
  equal 
  volume 
  to 
  the 
  carbon 
  dioxid 
  absorbed, 
  

   so 
  that 
  green 
  plants 
  during 
  sunlight 
  not 
  only 
  keep 
  the 
  proportion 
  of 
  

   carbon 
  dioxid 
  down, 
  but 
  actually 
  become 
  aerating 
  agents 
  by 
  reason 
  of 
  

   their 
  contributions 
  of 
  oxygen. 
  Hence 
  it 
  has 
  been 
  the 
  current 
  belief 
  

   that 
  aquatic 
  plants 
  are 
  necessary 
  to 
  furnish 
  the 
  oxygen 
  needed 
  by 
  

   aquatic 
  animals 
  and 
  to 
  remove 
  from 
  the 
  water 
  the 
  carbon 
  dioxid 
  

   injurious 
  to 
  the 
  animals. 
  

  

  In 
  1890, 
  however, 
  Seligo 
  indicated 
  that 
  the 
  importance 
  of 
  the 
  aera- 
  

   tion 
  role 
  of 
  aquatic 
  plants 
  has 
  probably 
  been 
  exaggerated. 
  

  

  For, 
  as 
  is 
  well 
  known, 
  plants 
  need 
  in 
  their 
  life 
  processes 
  not 
  only 
  the 
  nourishing 
  

   carbon 
  dioxid, 
  but 
  like 
  all 
  other 
  living 
  things 
  oxygen 
  also, 
  and 
  while 
  the 
  excretion 
  

   M 
  oxygen 
  takes 
  place 
  only 
  in 
  sufficient 
  light, 
  the 
  absorption 
  of 
  oxygen 
  goes 
  on 
  con- 
  

   tinuously. 
  If 
  then 
  the 
  oxygen 
  content 
  of 
  water 
  rich 
  in 
  plants 
  must 
  indeed 
  be 
  

   greater 
  by 
  day, 
  so 
  is 
  it 
  for 
  the 
  same 
  reason 
  much 
  the 
  less 
  by 
  night. 
  At 
  the 
  same 
  

   time 
  equalization 
  of 
  gases 
  must 
  take 
  place 
  very 
  rapidly 
  in 
  the 
  comparatively 
  shallow 
  

   shore 
  region 
  of 
  the 
  lake 
  basin, 
  not 
  only 
  by 
  access 
  of 
  the 
  outer 
  air, 
  especially 
  through 
  

   wave 
  motion, 
  but 
  also 
  by 
  diffusion 
  within 
  the 
  water 
  mass 
  itself; 
  and 
  just 
  as 
  the 
  

   assumption 
  that 
  forest 
  air 
  must 
  be 
  richer 
  in 
  oxygen 
  than 
  the 
  air 
  in 
  the 
  larger 
  cities, 
  

   for 
  instance, 
  has 
  been 
  shown 
  by 
  careful 
  air 
  analysis 
  to 
  be 
  erroneous, 
  so 
  can 
  the 
  

   oxygen 
  content 
  of 
  the 
  shore 
  water 
  rich 
  in 
  plants 
  be 
  scarcely 
  different 
  from 
  that 
  

   which 
  is 
  free 
  from 
  plants. 
  (Seligo, 
  1890, 
  p. 
  47.) 
  

  

  Oxygenation 
  of 
  the 
  superficial 
  layers 
  of 
  water 
  is 
  accomplished 
  b\' 
  

   mechanical 
  admixture 
  of 
  air 
  through 
  the 
  action 
  of 
  waves, 
  tributarj^ 
  

   streams, 
  and 
  rainfall, 
  so 
  that 
  the 
  upper 
  2 
  meters, 
  over 
  the 
  entire 
  sur- 
  

   face 
  of 
  the 
  lake, 
  is 
  practically 
  saturated 
  with 
  the 
  atmospheric 
  gases. 
  

   Oxygen 
  thus 
  absorbed 
  from 
  the 
  air 
  has 
  been 
  usually 
  thought, 
  as 
  by 
  

   Seligo, 
  to 
  diffuse 
  with 
  great 
  rapidity 
  into 
  the 
  deeper 
  layers 
  of 
  the 
  

   water, 
  but 
  Hoppe-Seyler 
  (1896, 
  p. 
  15) 
  has 
  measured 
  the 
  rate 
  of 
  diffu- 
  

   sion 
  of 
  oxygen 
  into 
  motionless 
  water 
  from 
  the 
  atmospheric 
  air 
  and 
  has 
  

   found 
  it 
  extremely 
  slow 
  and 
  wholly 
  inadequate 
  to 
  account 
  for 
  the 
  

  

  