Limiting factors for phytoplankton

Factors affecting the growth of phytoplankton in running waters include all the same variables that limit algal growth in lakes, such as light, temperature, and nutrients. However, discharge regime has a profound influence over river phy-toplankton, and the influence of light and nutrients differ in some ways from what is seen in standing waters. In addition, adjacent stagnant waters are critical to the establishment of river phytoplankton, through their influence upon the size of the inoculum. This can be of considerable importance, especially when residence time of the water mass is short enough to limit the buildup of populations.

An inverse relationship between river discharge and phytoplankton abundance is perhaps the most common finding of detailed investigations of river phytoplankton (Decamps et al. 1984, Filardo and Dunstan 1985, Sullivan et al. 2001). As a mass of water moves downstream and the entrained plankton multiply, one expects maximal abundances to be associated with a water mass that is traveling slowly and is uninterrupted over a long distance. Talling and Rzoska (1967) estimated that a water mass traversed the 357 km section of the Blue Nile between Sennar Reservoir and Khartoum in 40 days at low flows, but required only 2 days at high flood. Since phytoplankton populations are capable of a maximum of about 1-2 doublings per day, the consequences for eventual population size are considerable.

Using a data set that included 345 sites on large rivers and 812 lakes and impoundments within the continental United States, S0balle and Kim-mel (1987) concluded that rivers and lakes occupy two ends of a continuum, and impoundments fall in-between. Along the gradient from rivers to impoundments to lakes, residence time increased (mean values of 18.4, 528.5, and 1073.5 days, respectively), transparency increased, total P declined, and phytoplankton counts increased severalfold. Interestingly, water residence time appeared to act as a threshold factor, being of great importance at values <75-100 days, and of little importance when residence time was longer.

Although the usual effects of high flows are dilution and downstream transport, under certain circumstances floods might augment river plankton by washing in populations from stagnant areas. The lower Orinoco and several of its tributaries comprise a large tropical river system with fringing floodplain regions that are in contact with the river for up to 180 days in wet years. Lewis (1988) found that primary production per unit volume was greatest during the period of low water, but nonetheless was quite low, due to a combination of light limitation and the short residence time of river water. Total phytoplankton transport exhibited a minimum just as discharge began its seasonal increase and was maximal at high water. Lewis concluded that flushing of backwaters in or adjacent to the river channel accounted for increased transport during the rise of flood waters, and the flood-plain contributed phytoplankton during the flushing and draining periods associated with peak and declining flood stages.

In a large river with ample nutrients and a transit time long enough to permit multiplication of phytoplankton, it is likely that the phyto-plankton is limited by light via interactions among turbidity, depth, and turbulence. If the water column mixes to a depth greater than the photic zone, then an individual cell will spend part of the day at light levels too low to support photosynthesis (Figure 6.15). Cole et al. (1991) estimated that the average phytoplankton cell in the Hudson River would spend from 18 to 22 h below the 1% light level. Rather than growing, the cell would be expected to lose biomass. This is a real puzzle, because phytoplankton biomass does increase during the spring and summer. One possible explanation is that phytoplankton blooms originate only in river sections <4 m in depth (Cole et al. 1992). Similarly, Lewis (1988) reasoned that the bulk of phytoplankton biomass transported in the lower Orinoco system originates from stagnant waters in or adjacent to the channel, because once in the main channel, phytoplankton spent too little time at light levels sufficient for growth.

Turbidity and depth of mixing will of course vary from place to place and with seasons, greatly affecting the opportunities for growth of phytoplankton populations. Phytoplankton productivity was suppressed to a greater degree in the whitewater Apure River than in the black-water Caura River, corresponding to differences in light penetration (Lewis 1988). Depth of the euphotic zone varies seasonally with the sediment load, and also due to self-shading when phytoplankton are abundant. In the Lot River of south-central France, the maximum depth at which photosynthesis can occur ranges between 0.7 and 5.3 m (Decamps et al. 1984), depending upon season. In the Blue Nile, passage of the flood crest reduces Secchi disk readings to zero (Rzoska et al. 1952). Reservoirs usually enhance the conditions for phytoplankton growth because greater water clarity results from settling of sediments. Under these conditions, self-shading by the plankton replaces sediments in suspension as the principal limitation to light penetration.

Nutrient limitation of river phytoplankton does not appear to be usual in free-flowing rivers. Downstream transport and light typically are overriding variables, and nutrient concentrations in rivers, and especially in lowland rivers, often are considerably higher than in lakes.

FIGURE 6.15 Schematic diagram comparing effect of depth of mixing on primary production in phytoplankton of a lake versus a river. In a lake (a), establishment of a temperature barrier between surface and deep waters restricts mixing to the upper few meters. In a river (b), temperature stratification is impeded by turbulence of flow, and the water column typically mixes from top to bottom. Depths of 5-20 m are common in large rivers. Rivers often carry substantial sediment loads, restricting light penetration to, at best, the upper 1-2 m.

FIGURE 6.15 Schematic diagram comparing effect of depth of mixing on primary production in phytoplankton of a lake versus a river. In a lake (a), establishment of a temperature barrier between surface and deep waters restricts mixing to the upper few meters. In a river (b), temperature stratification is impeded by turbulence of flow, and the water column typically mixes from top to bottom. Depths of 5-20 m are common in large rivers. Rivers often carry substantial sediment loads, restricting light penetration to, at best, the upper 1-2 m.

In their survey of more than 100 sites on lakes, impoundments and rivers, S0balle and Kimmel (1987) observed a positive relationship between P concentrations and phytoplankton biomass, a well-established trend in lakes. However, river phytoplankton abundance was several times lower than would be expected based on P availability, and the relationship also was more variable, suggesting that factors other than nutrients exerted primary control. A similar pattern was observed by Basu and Pick (1996), who found a positive and significant correlation between chlorophyll and total P concentrations measured in 31 rivers in eastern Canada. Koch et al. (2004) determined experimentally that light and nutrients could limit phytoplankton grown in the Ohio, Cumberland, and Tennessee rivers. When irradiance was below a threshold, light limitation was frequently observed for the phytoplankton of the Ohio River, which is more turbid than the regulated Cumberland and the Tennessee. When irradiance was higher, P limitation was obtained for the Cumberland phytoplankton, while N and P were colimiting in the Tennessee and silicate was limiting in the Ohio River. In the River Rhine, a spring bloom of diatom-dominated phytoplankton reached very high abundances until dissolved silicate became depleted, which led to a population collapse (Vansteve-ninck et al. 1992).

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