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Reading 1.2

19 January, 2016 - 18:01

Summary of key ideas and findings from Hodgkiss (1984) 'Seasonal patterns of intertidal algal distribution in Hong Kong'

The condensation of this paper was prepared by the course developer for BIO S301. I am sure that you won't mind, as this summary shortens your reading by about eight pages!

Introduction
The paper by Hodgkiss is very good; it's a pioneering paper on intertidal macroalgae of Hong Kong that demonstrates that although Hong Kong is technically in the tropics, its climate is not tropical.

Some biologists seem unsure about what label to give Hong Kong's climate and while happy to describe it as monsoonal in one sentence, they feel they should also acknowledge that 'many, climatologists classify Hong Kong's climate as subtropical' and state this on the same page as they mention monsoonal climate. Some also offer the suggestion that the terms 'wet season' and 'dry season' might be more appropriate for a biologist (Dudgeon and Corlett 2004, 20). This can be confusing for readers. Other biologists are less unsure, and are confident to describe Hong Kong as having a subtropical, highly seasonal monsoonal climate (e.g. Kong and Ang 2004). Geographers and geomorphologists are also inclined to emphasize the subtropical nature of the local climate (Owen and Shaw 2001.).

The paper from Hodgkiss (1984) as summarized here gives a very clear set of biological evidence that the seasonality displayed by most intertidal algae supports the notion that Hong Kong's climate is subtropical; a point again dramatically demonstrated by winter chill shock events which killed tropical ecotypes of Avicenniamarina mangroves (Maxwell 2001) and Sonneratia spp, another tropical mangrove (Kwok et al. 2008). Main findings

  • Intertidal algal distribution on Hong Kong shores shows that exposure to strong sunlight is the major factor responsible for a clear pattern of seasonality in which most species disappear during the summer (late May, June, July, August);
  • Daily duration of bright sunshine varies from 1.4 hours in March to 7.4 hours in July;
  • The tides in the South China Sea are mainly diurnal.
  • Hodgkiss's data pool is set out in two tables you should examine closely:

Type (division)

Total for year

Summer spp:

all summer (1-3 months)

# absent during summer (%)

Table 1.1 Algal species numbers recorded and % absent during summer months

Green algae (Chlorophyta)

17

3(2)

12(70.5)

Brown algae (Phaeophyta)

20

1(3)

16(80)

Red algae (Rhodophyta)

19

5(1)

13(68)

Blue-green algae (Cyanophyta)

2

0(2)

0(0)

All species total

58

9(8)

41(70.6)

 

Habitat type, within hard shore

(not sandy or muddy)

No. of spp

Table 1.2 Number of algal species at different habitats in relation to season (mouth of the year)
 

Oct-May

June-Sept

% decrease in summer

Exposed

51

12

76

Sheltered

30

7

76

Estuarine

20

8

60

Oceanic

54

13

76

Exposed/estuarine

13

5

62

Exposed/oceanic

48

11

77

Sheltered/estuarine

12

3

75

Sheltered/oceanic

26

4

85

 

Points for thought

  • Summer tides and sun: illumination and desiccation on the seashore. During summer in Hong Kong the lowest ebbs of the tide (low tide times) occur between 2 pm and 6 pm. During the cooler weather between November and April the ebbs tend to be between 2 am and 6 am, i.e. when it is cool and approaching sunset. (You might be interested in tides and these can be easily accessed by visiting the tide tables provided by the Hong Kong Observatory. Visit: www.hko.gov.hk or www.weather.gov.hk . Or, if you prefer, you can purchase a booklet (bilingual) showing tide tables for seven different coastal locations around Hong Kong.) Thus, in summer, the intertidal algae experiences strong afternoon sun for a significant period of time. The temperature and drying (desiccation) effect can be severe. Only nine out of the fifty-eight algae survived for the entire summer! Those on the lower shore experienced less sunset (as you would expect from the daily tidal dynamics) but even short periods of extreme illumination, desiccation and 'slow cooking' at summer temperatures on exposed rocky shore surfaces can cause algal mortality. This has been known for a long time in algal biology (Chapman 1962; Hock 1982).
  • We may need more studies to distinguish the slight survival differences due to the relative importance of salinity (oceanic saline waters vs estuarine less saline waters), desiccation vs temperature effects and illumination vs temperature effects.

Perhaps you might like to do some follow up research on intertidal algae of Hong Kong. You will have the opportunity to examine this paper as Reading 4.3 soon.