Tropical Diatoms of Costa Rica
REU Research

The Importance of Solute Input and Canopy Cover on the Diatom Community Composition in Neotropical Streams, Costa Rica

Britt Carlson

St. Olaf College, Northfield, Minnesota
N.S.F. Research Experiences for Undergraduates program, La Selva Biological Station site grant

Britt in action!

Why study canopy cover and diatoms?

In neotropical streams flowing through primary and secondary forest, riparian canopy cover creates dense shading over most or all of the streams. The lack of light limits primary production by benthic algae (Paaby and Goldman 1992). However, even in dense canopy cover, patches of sunlight can break through the canopy and reach the forest floor or streambeds.

Sun flecks and sun patches may last only a short time, but, because of their high photon flux densities (a measure of light intensity), they can contribute considerable fractions of total daily irradiance to an area. Sunpatches may be important for primary production in forest streams.

Goals and Objectives

In the streams at La Selva, solute levels (Pringle and Triska 1991) and micronutrients (Pringle et al. 1986) directly affect algal accrual. In addition, high light conditions in the high and low solute streams seem to have a markedly different species composition (Bixby et al. 2003).

This study aims to understand how solute levels and canopy cover affect diatom community composition in neotropical streams. Because individual diatom species often have narrow environmental tolerances, species assemblages present in a given habitat are indicators of the abiotic (i.e., temperature, pH, nutrients) and biotic (i.e., grazing) environment (Lowe and Pan 1996). Streams with similar nutrient conditions should share a characteristic algal community. Additionally, because of the limiting nature of light, decreases in canopy cover should significantly increase the diatom density in a given area.

Methods

Diatom samples were collected from a known surface area on rocks in various streams within La Selva Biological Station. Ten low solute sites, eight high solute sites, and one P-enriched site were selected for this study. Those streams with solute input had higher phosphorus levels, but similar temperatures and turbidity compared to the low solute streams. At each stream site, three samples were collected as replicates. Current velocity, water depth, and canopy cover (with a spherical densiometer) were also measured.

The Lab Work

Diatom samples were processed in the laboratory. Each sample was processed using nitric acid (60%) and rinsed six times with distilled water to remove oxidation by-products. A subsample was then placed on a cover slip and allowed to dry. Coverslips were mounted on microscope slides using Meltmount ™. Samples were examined at 1000x magnification using a brightfield microscope. 200 diatoms were enumerated and identified to genus level (naviculoid taxa were counted by size class) along transects on the microcope slide.

Results and discussion

The results of this study indicate that community differences between neotropical streams with forested watersheds are driven primarily by geothermal activity, rather than canopy cover.

• Diatom densities in high solute streams were not significantly greater than in low solute streams (p = 0.5782). My streams ranged from 65-98% canopy cover. All of the sites in this study, then, can be considered low light conditions. The range of canopy coverage presented here may be too narrow to see differences in cell density and community composition. Most studies have used very coarse scales of light (high/low). The inability to measure changes in algal communities at a fine scale is not surprising due to the many challenges of measuring light microclimate ( Brown and Jennings 1998).

• But, while density of diatom cells in the two stream types (high and low solute) was not significantly different, differences in community composition did exist. Simpson's Index of diversity was calculated for all sites. High solute sites averaged 0.90 (n = 9); low solute sites had significantly lower community diversity, averaging 0.83 (n = 10, p = 0.0195) (Fig. 1).

• 

  Fig. 1. Average Simpson's Index of diversity for high solute and low solute streams (n = 19).

  In total, 43 genera were identified, with specific genera are characteristic of high or low solute streams. When comparing the genera that comprise at least 1% of the valves enumerated in a particular stream type (n = 23), there are 10 genera that are found significantly more often in one stream type versus the other (Table 1). Achnanthidium, Bacillaria, Cocconeis, Seminavis, Nitzschia, and Planothidium are abundant in high solute streams. Eunotia, Frustulia, Hippodonta, and Navicula <10um in size are abundant in low solute streams. Certainly, a study done at the species level would reveal more precise indicators for stream type. I suspect that differences between particular genera to utilize nutrients are driving the differences in community composition among high and low solute streams.

Table 1. Genera that comprise >1% of the total valves counted from La Selva streams. Genera are classified as high solute, low solute, or shared based on the significant differences (p value) between the two stream types. Naviculoid taxa are divided into four size classes.

Implications

As low order streams are exposed to more light due to deforestation, it is important understand its effect on primary production and aquatic food webs. The streams at La Selva are primarily detritus-based (Rosemond et al. 2002) and increases in primary productivity could have dramatic effects.

In addition, the increasing anthropogenic inputs of nutrients will have an effect on the algal community, both in high and low light conditions. In low light the affect may be more at the community level (as shown by this study) while in high light there may be both a community change and an increase in algal biomass (Bunn et al. 1999). The nature of anthropogenic inputs is also important. Results from the study by Bixby et al. (2003) indicate that in low light, phosphorus (a common nutrient in fertilizers) may not change the community. Studies of the effects of other nutrients, particularly those that are common pollutants, would be appropriate.