Phytoplankton size fractions in Tolo Harbour, Hong Kong and their relative contributions to productivity

by Chan, Shue-shum

(Uncorrected OCR) Abstract of thesis entitled

"PHYTOPLANKTON SIZE FRACTIONS IN TOLO HARBOUR,

HONG KONG AND THEIR RELATIVE CONTRIBUTIONS TO PRODUCTIVITY"

Submitted by

CHAN SHUE SHUM (ft ~ 4-

B.Sc. (TAIWAN)

M.Phil. (HONG KONG)

for the DEGREE OF DOCTOR OF PHILOSOPHY

at the UNIVERSITY OF HONG KONG

in September, 1987.

"~'e'fM'''''''c~ ... '._ 'L~, ~~.

tJ"

ABSTRACT

Studies on a natural phytoplankton population in Tolo Harbour

based on four categories of size fractions and their relative

contributions to productivity revealed not only the persistant

dominance and superior productivity of the smallest size fraction

but also that phytoplankton primary productivity was inversely

proportional to size. The nannoplankton ?0 ~m) size fractions

were the most important component accounting for 57% of the annual

biomass (in terms of chlorophyll a concentration) at an inner

station T3; and 47% at an outer station T5.

In terms of carbon

uptake, the contribution of the nannoplankton to annual

productivity was also important at both inner and outer stations

(50.57% and 43.87% respectively). Conversely, the netplankton

contributed relatively lower percentages to biomass and

productivity, contributing 4.1% and 19.4% at stations T3 and T5 in

terms of chlorophyll a concentration and 5.41% and 20% at stations

T3 and T5 in terms of carbon uptake. The higher cell surface area

to volume ratio of the nannoplankton is thought to be one of the

major reasons for this.

All forms of nutrient content were high at both stations and

-N (like p) is no longer a growth-limiting factor in Tolo Harbour.

The high concentrations of Nand P should have supported a much

higher level of phytoplankton biomass; but it appeared that the

high Nand P concentrations suppressed further growth.

Productivity in the surface layer was generally higher than

in. the middle and bottom layers of water.

~ ..

The high productivity, especially at the inner station T3,

resulted in algal blooms. High nutrient concentrations of both N

and P, favourable ambient temperature and light, long retention

time as a result of restricted tidal exchange, absence of

upwelling, but presence of mixing enhanced by both movement of

incoming and outgoing vessels and by river runoffs, especially

during and after torrential rain, promoted 32 red tide incidents

in Tolo Harbour during the study period. The causes of most of

the algal blooms are extremely diverse and include hydrographic,

chemical and biotic effects on cell growth. The control of

destructive blooms (which have caused annual losses of millions of

dollars via fish-kill In cage-fish farming in Tolo Harbour) will

depend upon achieving an understanding of both their basic biology

and their ecology.

Red tide incidents could well be a warning signal of

deterioration in the aquatic ecosystem and indicate that concerted

action against further deterioration is necessary.

In Tolo Harbour, the outbreaks of red tides appeared to be

initiated shortly before the commencement of thermal

stratification.

Assimilation numbers of all four phytoplankton size fractions

were found to show a better relationship with temperature --

though they were not significantly correlated at the 95% level

than with light and salinity. The variations in assimilation

numbers in each size fraction were related to environmental

fluctuations. The lower assimilation number value given by the

smallest size fraction ?0 ~m ) was probably due to large numbers

of dead cells and detritus present in that size fraction which can

be reflected by the high phaeophytin value given by that size

fraction.

Minute eukaryotic cells ? ~m) are now receiving more attention in terms of their significant contribution to

productivity. However, further investigation of the physiology and ecology of this ultraplankton would promote a better understanding of energy transformations and recycling in the

oceans.