INFORMACIÓN GENERAL DE LOS

Small whole sporophytes of L, saccharina were grown in the laboratory for 20 days at ambient seawater temp­ erature (10 ^ C ) and ambient photoperiod (14 hours Light) with high levels of phosphate (3,0 pg-at P,1 ^ ), The control treatment contained the ambient nitroien concen­ tration of the seawater (0.5 pg-at N,1 ^') , In the

+Nitrogen treatments, the different nitrogen sources were supplied at 7.5 pg-at N.l ^ ,

The fronds increased in dry weight over the course of 20 days in all treatments (Table 4 viii) but this inc­ rease. was only significantly greater than the low-N control when nitrate or nitrite was added. The mean linear growth rate over 20 days (Table 4 viii) was similar bet-'/een all treatments and the frond surface area only increased sig­ nificantly above that of the control when nitrate was supplied at 7,5 pg-at N.l ^ .

The change in total internal N content of the meristem and mature tissue over the course of the experiment is

shown in Table 4 ix. In the controls the total N content of both tissue areas is reduced significantly over 20 days.

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Fie, 4 XXV, Linear ,e r o t h rate of the iron d ro-.m in lov;-;! oea'/ater ( :'in , day “ ) of L « saccharina

(0,5 pe-at h , 1 ~, Control) and v/ith added nitrati (7,5 pe-at N.l “ ) over the course of 20 days in

September, (hean + SS, 3 replicates/treatment) -{•Nitrate C on t r o 1 5 , Linear 3 r o ; t h Rate an,day 3 , -1 2

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The total N content was significantly higher than the

controls in all +N treatments (P<0,001) but the difference betv/een all +N treatments was not significant.

The. low-N controls appear, therefore, to be utilising internal N reserves to maintain growth rates similar to the +M treatment growth rates. Over the course of the experiment linear growth rate remains relatively constant in the controls (Fig. 4 xxv) but if the experiment had

been extended in time, it is suggested that growth rates 'f

would decline as internal reserves became further depleted. With supplementary N, the mean linear growth rate is not

raised significantly above the low-N controls. During |

the experiment the linear growth rate of +Nitrate

sporophytes shows an initial increase 0-8 days but levels y

off after this (Fig, 4 xxv) although the tissue continues

i

to accumulate N, and the final growth rate (16-20 days) is not significantly higher than during 0-4 days. Some factor must become limiting to growth and it is unlikely

•Vj,

to be phosphate concentration, photoperiod, seawater temp- ¥

erature or nitrogen concentration which are all high at this time. It is possible that the tissue has become

senescent and has only a very limited response to increased nitrate. ^ situ tissue-N content is largely determined by seawater N concentration (4 iii a ) , however in this experiment the tissue N content of the nitrate enriched

treatment is lower than predicted from the external con- " f centration ;

External Cone, of control and + nitrate treatment is %

1 60 in the ratio 1.0 : 15.0,

Tissue-N content of the control and the +nitrate treatment is in the ratio 1.0 : 8,4.

The ratio of the mean linear growth rate between the 3 ■ 58

control and +nitrate treatment ( — = 1.51) is considerably lower than predicted from either the seawater concentration or the tissue-M content. L. saccharine is, therefore,

not responding in September in the same way to increased or high N as it does in the Spring (4 iii c) suggesting some factor(s) limiting growth. As it is unlikely to be phosphate or nitrate concentration, seawater temperature or photoperiod, senescence is proposed as a possible factor limiting Laminaria response to apparently 'ideal' c on di ti on s.

DISCUSSION

The typical seasonal fluctuation of nitrate and nitrite in the seawater is well kno'wn but in contrast to some

locations (English Channel; Harvey, 1926) nitrate is never completely exhausted at the sampling sites as agricultural run-off and pollution continuously replenish supplies. Ammonium remains relatively high during the summer but concentrations approximate to minimum summer levels of

nitrate and it is the variation in nitrate which, therefore, determines the overal seasonal fluctuation in seawater

nitrogen. The tissue-H content of L . saccharina and L. digitata follows the seawater N concentration and is high in the winter and low during the summer,

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Laminaria spp. accumulate nitrogen in excess to growth requirements during the winter, indicated by the increasing internal organic and inorganic-K reaching a maximum content per unit dry weight in April. With adequate K (both

externally and internally) much of the nitrate taken up is retained (and is extractable in short-term uptake experi­ ments) as nitrate (i no rganic-N), Since the organic-N content also increases during the winter, a proportion of the inorganic-N taken up must be used for reduction and

amination and there is also some storage of M as non-protein and perhaps some protein K. The large remaining pool of inorganic-N may, therefore, represent a biophysical surplus to metabolic requirements.

The decline in seawater K in April occurs as accumula­ tion of N by Laminaria and other macroalgae (eg. A s c o p h y H u m no do sum (Black, 1948); Fucus spp, (Black, 1949)) reaches a maximum and phytop lankton density peaks at St. Andrews (Richardson, 1969). Total internal N drops sharply as growth rates continue to increase for a further month; inorganic-N is utilised most rapidly followed by organic (non-protein)-N as the conversion to protein and dilution by other algal constituents continues. The low external N and the depletion of tissue NC^ restricts the net synthesis of further low molecular weight organic-K compounds which, in turn, limits protein synthesis. This restriction on protein synthesis resulting from the reduced availability of li (both internally and externally) may cause a decrease in linear growth rates after a lag period of one month; this

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must be the tine required for utilisation of stored reserves, This decline in growth, in turn, limits the utilisation of

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