Hydrological and habitat controls of algal biodiver sity and biomass in a subtropical flood-pulsed wetland

Luca Marazzi, MScPh.D. Candidate

Department of GeographyUniversity College London

INTECOL 2013 – 19 th August 2013

Study area: the Okavango Delta

1. Introduction

Flooded area: 2,500–4,000 km2 (Feb-Mar) to 6,000–12,000 km2 (Aug-Sep)

Ramsar and UNESCO Heritage site

Source: MODIS satellite image.

Microalgae, base of aquatic food webs

Ecosystem services. Provisioning: food, fiber and fuel. Regulating: climate regulation; water purification. Supporting: primary production; nutrient cycling; production of atmospheric oxygen

1. Introduction

Acknowledgements

� Anson Mackay, Viv Jones, Sophie des Clers, Lars Ramberg; Brian Whitton, David John, David Williamson and Chris Carter, Gina Clarke, Tom Davidson, Barbara Leoni, Letizia Garibaldi

� Royal Geographical Society, UCL Geography Department & Graduate School, UK DEFRA Darwin Initiative (funding forfieldwork and other expenses)

� Nqobizitha Siziba, Ponde Kauheva, Richard Mazebedi and Okavango Research Institute staff

� Responding to Climate Change, Campbell Harris Tutors, Greater London Tutors, Mouseion Limited (employers 2009-2013)

Main aims and key questions

A1)Present first ever estimate of algal biodiversity in the Okavango Delta

A2)Explain biomass and diversity patterns in relation to macrohabitats and environmental variables

Q1) Does algal biomass grow with increasing speciesrichness?

Q2) What is the shape of the diversity-biomass curve inthe different macrohabitats?

1. Introduction

The Okavando Delta: sampling campaigns

= Algal sampling

R

H

LE

Campaign 1 : I. Sep. 2006 (Flood Recession ); II. Apr.-May 2007 (Flood Expansion ); III. Jul.-Aug. 2007 (High Water )

Campaign 2 : I. May 2009 (Flood Expansion ); II. Jul-Aug. 2009 (High Water ); III. Oct. 2009 (Flood Recession ); IV. Feb. 2010 (Low Water )

Upper Panhandle (UPH); Lower Panhandle (LPH); Xakanaxa (XAK ); Boro (BOR); Santantandibe (SAN). [Ngami Lake (NGA), Thamalakane (THA)]

2. Methods

Macrohabitats sampled

Open Water (main channel) & Marginal Vegetation (e.g.:Typha, Phragmites, Vossia) Floodplain

Open Water, Sedgeland and Grassland

1)Delta-scale

2)Within-floodplain

For comparisons across all samples

For investigation of floodplain dynamics

2. Methods

Water samples for algae + limnological variables

Algal counts – 132 samples (49,975 algal units)

Regions Bac Chl Chr Cry Cya Eug Pra Pyr Uni Xan

UPH (N) 4,407 1,095 14 133 214 36 0 12 13 7

LPH (N) 2,560 839 30 281 132 34 0 9 8 6

XAK (E) 2,196 1,108 230 365 192 102 0 67 1 11

BOR (S) 6,190 14,068 81 1,404 2,277 762 0 115 182 95

SAN (S) 3,918 4,311 21 617 605 647 1 72 60 28

(NGA - SW) 179 217 0 0 17 0 0 0 0 1

Total 19,450 21,638 376 2,800 3,437 1,581 1 275 264 148

•About 400 algal units (cells, colonies, filaments, coenobia) counted in each sample

•Biovolumes estimated using cell measurements and geometric formulas

[Bac= Bacillariophyta; Chl=Chlorophyta; Chr=Chrysophyt a; Cry=Cryptophyta; Eug= Euglenophyta; Pra= Prasinophyta; Pyr=Pyrrophyta; Uni=Unid entified; Xan=Xanthophyta]

2/3. Methods / Results

Species richness per algal phylum

Phylum N Species N Genera

Bacillariophyta (B) 113 37

Chlorophyta (Chl) 283 88

Chrysophyta 7 6

Cryptophyta (Cr) 11 3

Cyanophyta (Cy) 28 22

Euglenophyta (Eu) 37 6

Prasinophyta 1 1

Pyrrophyta (Py) 8 7

Xanthophyta 7 4

Total 495 174

Cryptomonas sp. (Cr)

10 µm

Phacus longicauda (Eu)

Micrasterias pinnatifida (Chl) (© Chris Carter )

Anabaena sp. (Cy)

20 µm

3. Results

20 µm

Stauroneis phoenicenteron (B)

30 µm

20 µm

Peridinium sp. (Py)

10 µm

New variety: Cosmarium pseudosulcatum var okavangicum

Williamson D.B. and Marazzi L.. 2013. A new Cosmarium (Chlorophyta, Desmidiaceae) variety from the Okavango Delta, Botswana. Quekett Journal of Microscopy 42, 35:37.

(© David Williamson )

3. Results

Diversity-biomass relationship

Floodplain sites: higher species richness, biovolumes and Chl a

130 samples 2009-2010 samples

3. Results

Biovolumes: Mean=8.103 mg/L(0.176 - 90.41 mg/L)

Temperature and habitat vs species richness

R2adj=0.286; p=0.000

3. Results

Key factors for species richness:

1.Temperature (+)

2.DO (-)

3.TN (-)

4.depth (-)

Facilitation

3. Results

Higher biomass

Factors driving diversity and biomass

Competition

Key factors for biovolume:

1.Cations (+), 2.temperature (+),3.species richness (+)

Algal cell length across regions / habitats

3. Results

•On average smaller algae in Southern Delta

•In some floodplains (Bororegion) larger algae than in other habitats

Diversity-biomass across macrohabitats

MV=Marginal Vegetation

F=Floodplain

OW=Open Water

Plankton: Competitive displacement

Benthos: FacilitationSee: Passy, S. I. and P. Legendre (2006). "Are algal communities driven toward maximum biomass?“

3. Results

“Facilitation in the benthos delays the negative effect of competition , which, together with the

weaker response to top-down controls, permits more species to coexist and produce maximum biomass”

Macrohabitats for algae (Northern Delta)

Upper and Lower Panhandle: Open Water and Marginal Vegetation

Upper Panhandle with Papyrus

Eunotia pectinalis

20 µm

Eunotia sp.

20 µm

Synedra sp.

30 µm

Cryptomonas sp.

10 µm

Here water velocity is higher, high profile diatomsare very abundant

Eunotia rhomboidea

20-30 µm

3. Results

Floodplains (Boro, Santantandibe): Open Water and Floodplain habitats (Sedgelands and Grasslands)

Floodplain with sedges and grasses (e.g. Schoenoplectus and Cynodon)

5 µm

Cosmarium haynaldii

25 µm

Micrasterias tropica Mougeotia sp.

20 µm

Macrohabitats for algae (Southern Delta)

Scenedesmusfalcatus

Both planktonic and benthic algae attached to sand / sediments or plants

15 µm

Euastrum bidentatum(© Chris Carter )

3. Results

Conclusions

1. 495 species of algae + epiphytic diatoms found by Mackay et al. (2012) = total 610 species (Pantanal: 337 spp.; Kakadu National Park: >700 spp.)

2. Habitat diversity generated by the slow periodic flood-pulse creates more heterogeneous conditions in the floodplains; thus the higher algal species richness (whichdrives algal biomass) .

3. Mechanisms: Facilitation / niche overlap occurs in the Floodplains (greater habitat diversity, e.g. in 3D biofilms); competition / niche complementarity prevail in the Open Water (less habitat diversity)

Ongoing and possible future work

• Multivariate analyses to interpret relative roles of all environmental variables (e.g. conductivity, DO, TN, TP) in shaping algal communities (cluster analysis / ordination & response curves for key taxa).

• Exploring synergies with other projects, e.g. “The Future Okavango” ; funding applications for raising awarenesswork on algae with local communities.

• Post-PhD : Further taxonomic analyses? Study of food-webs ? Comparative biogeographic research acrosstropical wetlands? Distribution of desmids via birdmigration ?