FIGURE 5.1 Examples of submersed vascular plants that occur in estuarine systems: (a) Myriophyllum...

FIGURE 5.1 Examples of submersed vascular plants that occur in estuarine systems: (a) Myriophyllum spicatum (photo: O. Pedersen), (b) Stuckenia pectinata (photo: O. Pedersen), and (c) Vallisneria americana (photo: Seafavorites.com) are freshwater plants that are able to tolerate low salinities (<15), while (d) Thalassia testudinum (photo: O. Pedersen), (e) Zostera marina (photo: P.B. Christensen; NERI), and (f) Halophila ovalis (photo: P. Lavery) are seagrasses that thrive in full-strength seawater.

ESTUARINE ECOLOGY, Second Edition. John W. Day JR, Byron C. Crump, W. Michael Kemp, and Alejandro Yánez-Arancibia. Copyright © 2013 by Wiley-Blackwell. All rights reserved

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FIGURE 5.2 One of the most serious physiological challenges for submersed vascular plants is to supply rhizomes and roots with sufficient oxygen to support aerobic metabolism and prevent invasion of anaerobic metabolites from the sediment. The figure shows how internal plant oxygen and gaseous sulfide, measured with microelectrodes, interact over a diel cycle in a T. testudinum bed in Florida Bay, USA, during a seagrass die-off period. The oxygen within the plant meristem changed from supersaturation (i.e., above the oxygen partial pressure of the atmosphere indicated by the stippled line) during daytime to very low levels at night, when water column oxygen content was low. When plant oxygen is low, poisonous sulfide can invade from the sediment into the plant as seen shortly after midnight on this occasion. Source: Redrawn after Borum et al. (2005).


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FIGURE 5.3 The probability of finding eelgrass at a cover (in % of bottom area) of greater than 0, greater than 2, greater than 25, greater than 50 and, greater than 75% at different depth intervals and as functions of light availability (in % of surface photon flux density, PFD), wave exposure (in relative units determined by fetch, wind direction and velocity, and water depth; Krause-Jensen et al., 2003) and salinity. Source: Redrawn after Krause-Jensen et al. (2003).


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FIGURE 5.4 Frequency distribution of biomass and production compiled for a large number of seagrass species. Redrawn after Duarte and Chiscano (1999).


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FIGURE 5.5 Colonization depth for seagrass communities distributed worldwide versus light attenuation coefficient reflecting water turbidity. The relationship is described by the equation log(Zc) = 0.26 – 1.07 · log(k), where Zc is the colonization depth and k is the light attenuation coefficient. Redrawn after Duarte (1991).


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FIGURE 5.6 Temperature dependency of light saturated net photosynthesis for eelgrass collected in winter (February), spring (April), and late summer (August). Optimum temperature for light saturated net photosynthesis varied from 20 to 25°C depending on the season. Source: Redrawn after Staehr and Borum (2011).


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FIGURE 5.7 Net photosynthetic rates relative to maximum net photosynthesis of two macroalgae species: Ulva lactuca and Saccharina latissima (formerly Laminaria saccharina) and two seagrass species: T. testudinum and Z. marina at normal seawater pH but varying availability of inorganic carbon (DIC in mM). Normal (1) and double (2) seawater DIC levels are indicated on the x-axis. Redrawn after Beer and Koch (1996).


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FIGURE 5.8 Ratio between current velocity measured inside and outside an eelgrass bed during periods with different aboveground biomass. The ratio between current velocities clearly declined with increasing biomass, reflecting the damping effect of the canopy. Redrawn after Hasegawa et al. (2008).


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FIGURE 5.9 Maps of (a) turbidity and (b) oxygen saturation created from measurements across and outside a robust bed of S. pectinata (sago pondweed) in Chesapeake Bay. The solid black line represents the perimeter of the bed, while the dotted area represents land. Turbidity was measured in NTUs (nephalometric turbidity units), a relative measure of underwater light transmission. The maps show a reduction in turbidity and increase in oxygen saturation with distance into the plant stand, which reflects the substantial impact submersed plant beds can have onwater quality in shallow environments. Source: Redrawn after Gruber and Kemp (2011).


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FIGURE 5.10 Sediment profiles of dissolved ammonium in vegetated sites consisting of mixtures of Z. marina and H. wrightii (closed circles) and unvegetated sites (open circles). Error bars are the standard error. The higher level of porewater ammonium in the seagrass bed reflects the retention and recycling of organic matter produced by the community or trapped from sources outside the plant bed. Source: Redrawn after Kenworthy et al. (1982).


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FIGURE 5.11 The abundance of the diving duck, Redhead (in % of the total count for North American breeding grounds), resting during winter in upper Chesapeake Bay versus the relative abundance of submersed vascular plants at upper Bay sites. The number of resting Redheads correlated well with plant abundance during the period from 1961 to 1975. With gradually decreasing plant abundance up through the 1960s and 1970s, Redhead abundance declined in this area and the ducks moved to areas with eelgrass further south in Chesapeake Bay. Source: Redrawn after Kemp et al. (1984).


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FIGURE 5.12 Abundance (number of individuals per meter square) and species richness (number of species) of finfish from May through September at an unvegetated and a vegetated site in the Choptank River, Chesapeake Bay. In the vegetated area, finfish were more abundant and more species were found compared to those in the unvegetated area, especially in August and September. Redrawn after Kemp et al. (1984).


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FIGURE 5.13 Decadal trends in gains and losses of seagrass beds recorded world wide. (a) Decadal gains and losses of area covered by seagrass and (b) number of sites where changes (decreasing, increasing, or no change) have been recorded. The records show higher areal losses relative to gains and a relatively larger number of sites exhibiting decreases than increases of seagrass area since the 1950s. Redrawn after Waycott et al. (2009).


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FIGURE 5.1 Examples of submersed vascular plants that occur in estuarine systems: (a) Myriophyllum...

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