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PHOTOSYNTHETIC RESPONSE OF LAKE PLANKTON TO COMBINED NITROGEN ENRICHMENT 1
Author(s) -
Lean D. R. S.,
Murphy T. P.,
Pick F. R.
Publication year - 1982
Publication title -
journal of phycology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.85
H-Index - 127
eISSN - 1529-8817
pISSN - 0022-3646
DOI - 10.1111/j.1529-8817.1982.tb03217.x
Subject(s) - photosynthesis , biology , nitrogen , nutrient , trophic level , plankton , botany , nitrogen cycle , environmental chemistry , zoology , ecology , chemistry , organic chemistry
The complex interplay between photosynthesis and the uptake of nitrogen was investigated in samples from five lakes of different size and trophic state. When enriched with 15 NH 4 + , the photosynthetic rate was often reduced for 4–5 h in samples believed to be nitrogen deficient. This implies that energy was reallocated from photosynthesis to the uptake and assimilation of N. Stimulation in C uptake at low levels of NH 4 + enrichment was followed by a progressive decline with further NH 4 + enrichment. On other occasions when ambient NH 4 + was undetectable, nutrient regeneration by zooplankton supplied a significant fraction of the required nitrogen. At these times and when the plankton had sufficient available N, there usually was no change in photosynthetic rate with either NH 4 + or NO 3 − enrichment. Typically, little NO 3 − was taken up and no photosynthetic response was observed. On two occasions, however, the uptake of NO 3 − was significant due to high NO 3 − and low NH 4 + levels early in the season. At one of these times there was a reduction in photosynthesis with NO 3 − enrichment. A further complication was observed when photosynthesis decreased with NH 4 + enrichment but increased with NO 3 − enrichment despite negligible NO 3 − uptake. These observations illustrate that the complex metabolism of these two nitrogen sources is not fully understood. At optimum light intensity, C:N uptake ratios, even under NH 4 + enrichment, are only sufficient to maintain the cellular C:N ratio unless much of the fixed C is respired or excreted. Three observations suggest that photosynthesis and N uptake are not coupled, (i) Photoinhibition of C uptake, but not N uptake was observed when low light adapted populations are exposed to high light conditions, (ii) The light intensity for maximum N uptake was slightly less than that for carbon. (iii) Dark N uptake was always near 50% of the maximum rate in the light whereas the C uptake was near 2% of P opt. Certainly, there is an interconnection because dark C uptake was enhanced by NH 4 + enrichment.