During the oscillatory movement of the water 

 mass in the Deep Hole, cold bottom water is sometimes 

 forced into Long Point Bay. At such times the level 

 of the thermocline in the vicinity of the point (station 

 17) varies greatly and remarkable temperature gradients 

 result. On one occasion the thermocline rose 3 meters 

 in 9 hours. At another time a difference of 6. 6° C. 

 was observed in one -half meter. The temperature of 

 the bottom water in Long Point Bay may thus fluctuate 

 widely during the summer and under the influence of 

 strong winds contributions from the lower levels in the 

 Deep Hole become mixed, lowering the surface tem- 

 perature throughout the bay. The low surface temper- 

 atures often recorded from this area may be largely due 

 to this. The rapid change in surface temperature is 

 shown at one station when a rise of 3° C. occurred in 

 22 hours. 



In view of frequent reports of strong currents in the 

 open lake, it was surprising to find that during the inter- 

 val of the investigations the maximum velocity recorded 

 was 0. 6 m.p. h. in 1929 and 0. 63 m.p.h. in 1928. 

 Nine observations taken during the fOTmer season in the 

 most unsettled portion of the lake (station 17) showed 

 a mean velocity of 0. 26 m. p. h. Here the water is 

 forced past Long Point following any alteration of the 

 lake level. It is probable that the natural eastward 

 flow in the lake rarely exceeds 0. 1 m. p. h. , except 

 near the river mouths. This movement is mainly con- 

 fined to the central axis of the lake, affecting mostly 

 the middle third of the water mass. The movement is 

 not continuous and may be halted or even reversed by 

 wind action. 



Current velocity was not always proportional to 

 wind velocity and the direction of the wind and cur- 

 rent did not always coincide. Sometimes winds of 

 short duration were directly opposite to the current. 



Two primary seiches, previously reported, were 

 observed in 1929. The period for the east -west oscil- 

 lation appears to be 14. 1 hours and for the north-south 

 oscillation 2. 7 hours. 



CHEMISTRY 



The chemical results for the years 1928 and 1929 

 agree so well that there can be little doubt that the rec- 

 ords for these years indicate ncwmal conditions in the 

 lake. The seasonal progression appears clear. 



After the spring overturn, chemical conditions 

 were no doubt uniform from top to bottom and free 

 carbon dioxide accumulated during the winter was 

 thoroughly distributed through the water mass. Active 

 phytoplankton production during the vernal flowering 

 season rapidly depleted the carbon dioxide in the sur- 

 face waters where it was exhausted by June. The in- 

 tensity of plant production was shown by this rapid 

 change in spite of the fact that processes of decom- 

 position would be accelerated by the rising temper- 

 ature. The shallow area of the lake evidently re- 

 sponds more quickly to changing climatic conditions 

 than the deeper zone and is also a region of greater 

 photosynthetic activity. 



Free carbon dioxide diminishes rapidly in the bot- 

 tom waters during the summer, being limited to the 

 two basins in August. The destruction of the thermo- 

 cline in the western basin resulted in increased bottom 

 oxygen and the elimination of carbon dioxide. Free 

 carbon dioxide was absent in the Great Plain Basin by 

 September. In seasons when winds might prevent the 

 formation of a thermocline in this area it is probable 

 that the free carbon dioxide would be found limited 

 to the Deep Hole after June. Water with free carbon 

 dioxide in the eastern basin had been restricted to a 

 zone within the 30-meter contour by September 1929. 

 Free carbon dioxide is present in the Deep Hole through 

 out the year. 



The oxygen content in Lake Erie is relatively high 

 and the extremes repcarted from other lakes never oc- 

 cur. The surface plant production, although extremely 

 rich, does not greatly supersaturate the surface layers. 

 Similarly bottom decomposition of organic materials 

 does not seriously influence the oxygen content, which 

 normally averages between 60 and 70 percent. 



An interesting exception was found at stations 41 

 and 42 in the vicinity of Rondeau. Unusually low 

 oxygen, low pH, and high carbon dioxide were recorded 

 here in 1929. The cause of these conditions has not 

 been established, but their occunence in the only large, 

 offshore, black mud area in the lake appears significant. 

 Two large patches of black mud, rich in organic mater- 

 ial, occur east and west of Rondeau, and the chemical 

 values indicate active processes of decomposition. 

 Elsewhere the chemical values were remarkably uni- 

 form, with gradual gradations into the deeper waters. 

 It may be added that the macroplankton in the black 

 mud area affords evidence that the low oxygen and 



176 



