Seagrasses 2

Light, Eutrophication & Foodwebs =

Why seagrasses are used to indicate coastal/estuarine system stability.

Light Limitation is the Principal Determinant of SAV Distribution

Light Environment

• Water causes light attenuation• Due to: suspended particles

(turbidity) and color (CDOM). Turbidity a fnt of phytoplankton and other “particles” (TSS)

• K = attenuation coefficient• Beer-Lambert Law:

Iz = Io e-zK

• Msr with secchi disk or PAR sensors

• PAR vs PUR

Z

I

Light Limitation is the Principal Determinant of SAV Distribution

Apparent Optical Properties vs Inherent OP’s

• AOP = Secchi depth (m) or attenuation coefficient (Kd) in 1/m

• Kd (AOP) depends on inherent optical properties (IOP’s)

• IOP’s depend on water quality

• IOP’s are a based on absorption (a) and scattering (b) of water+TSS+color+chl a

• Radiative transfer modeling provides the link between AOP and IOP’s of the 3 components of attenuation (TSS + color + chl a)

• From this relationship a direct link between water quality and light available to SAV can be made.

TSS

AOPIOP

•Eutrophication reduces light (phytoplankton and epiphytes),

•and increases sulfide-toxicity

Eutrophication

WHY SEAGRASS ONLY FOUND IN SHALLOW WATER?

15-25% of Surface Light (Io)

Seagrass loss causes sediments to be resuspended. Sediment resuspension causes water quality to decline -> less lightLess light means seagrasses are further prone to sulfide toxicityResults in ongoing losses of remaining seagrasses

Importance of WQ to SAVConceptual Model (Virnstein et al. 2000)

(TMDL)

$$$

Food Webs

Fewer Critters if no Grass Shrimp feed in Grassbeds

Reef Halos

Parrotfish and Surgeons

FLORIDA BAY 1997

Green-winged teal American wigeon

Redheads

• Migrating waterfowl• Important food sourcee.g. 5000 seeds in 1 duck Anatini (dabbling ducks) Aythyini (diving ducks) Cygnini (swans)

CONSUMERS OF SAV

Eutrophication?

+ Nutrients

BOTTOM-UP

+ algae/p’plankton

- seagrass

- light

TOP-DOWN

- mesograzers

+ small fish

- pred fishTrophicTrophiccascadecascade

TROPHIC CASCADES:•Jackson et al, Nature 2001

•Heck and Valentine, JEMBE 2006

Top-down and bottom-up control

Why we are concerned

3 papers to read

• William C. Dennison; Robert J. Orth; et al. 1993. Assessing Water Quality with Submersed Aquatic Vegetation BioScience, Vol. 43, pp. 86-94.

• Kenneth L. Heck Jr, John F. Valentine. 2006. Plant–herbivore interactions in seagrass meadows. Journal of Experimental Marine Biology and Ecology 330: 420–436

• Michelle Waycott, Carlos M. Duarte, et al. 2009. Accelerating loss of seagrasses across the globe threatens coastal ecosystems. Proceedings of the Natural Academy of Sciences (PNAS) vol. 106 pp. 12377–12381