13.0  Phytoplankton Productivity in Futch and Hewletts Creek

by

Virginia L Johnson
Center for Marine Science
University of North Carolina at Wilmington
Wilmington, NC

Introduction

            Phytoplankton are microscopic plants found in marine, estuarine and freshwater ecosystems.  Phytoplankton, like other plants, utilize sunlight to convert carbon dioxide into high-energy carbohydrates and release oxygen during the process of photosynthesis.  The rate at which these processes take place is known as primary production.  Collectively, phytoplankton are the foundation of food webs in water systems, providing a nutritional base for zooplankton and other commercially important shellfish and finfish. 

Typically, marine, estuarine and freshwater phytoplankton abundance is dependent on a number of physical environmental factors including, but not limited to, light, temperature, salinity and some function of nutrient availability.  Phytoplankton productivity and abundance has been shown to correspond to seasonal fluctuations in water temperature and day length throughout many east coast estuaries (Dame et al. 2000, Caffrey 2004, Mallin 1994).  Nitrogen, phosphorus, and iron, among other nutrients, have been shown to limit algal growth in freshwater, coastal and open ocean systems (Ryther and Dunstan 1971).  The biological characteristics of the system also play a role in regulating primary productivity, as phytoplankton abundance can be a function of zooplankton grazing (Mallin and Paerl 1994). 

            Tidal creek ecosystems are widespread and highly abundant along the Atlantic Seaboard and Gulf Coast.  Unlike many larger neighboring estuaries, tidal creeks systems do not necessarily follow a longitudinal river-ocean continuum and generally have a higher surface area to volume ratio than river-dominated estuaries.  Collectively, this could make their importance in material transfer and other ecological processes on par or even greater than larger estuaries in some regions (Mallin and Lewitus 2004).  

Unfortunately, tidal creek ecosystems are enduring changes as a result of a steadily increasing human population along the Atlantic Coast, including southeastern North Carolina.  Urbanization results in disturbances such as land clearing, application of fertilizers, discharge of human and animal waste and increased impervious surface coverage, which collectively act to increase nitrogen and phosphorus concentrations in neighboring surface waters and ground waters.  Nutrient over-enrichment, or eutrophication, can have profound effects on ecosystem processes by over-stimulating phytoplankton productivity and biomass accumulation leading to nuisance and toxic algal blooms (Cloern 2001).  Eutrophication is also often associated with increased biochemical oxygen demand and subsequent low dissolved oxygen problems (Mallin et al. in press), reductions in available light energy for benthic plants and changes in the plant community (Cloern 2001).  This can lead to changes in the natural function of tidal creeks as fish and wildlife habitat.  Trends show overall decreases in algal species diversity in streams with increasing urban land use usually due to factors including water chemistry (Paul and Meyer 2001).

Estuarine systems have a hydrological link to terrestrial landscapes and are thus subject to non-point source (NPS) runoff from the upland watershed.  While locations near the mouth of an estuary would be expected to be more closely characteristic of the coastal ocean, headwaters can receive an influx of materials from the upland watershed.  Chemical pollutants including nutrients, pesticides, and heavy metals bind to sediments from the terrestrial landscape and are introduced into water systems via NPS runoff.  In an urban landscape nutrient molecules have been shown to travel further distances downstream before removal from the water column, suggesting that normal nutrient removal efficiency can be greatly reduced (Paul and Meyer 2001).  Urban streams have displayed a greater tendency to suffer from low dissolved oxygen when compared to forested streams primarily attributed to increases in labile sources of carbon, or BOD, from the upland watershed (Paul and Meyer 2001). 

Two tidal creeks in southeastern North Carolina, Futch Creek and Hewletts Creek, will be principal subjects of the current study.  Both creeks receive anthropogenic nutrient loading, especially in upstream areas, and have been host to occasional algal blooms.  Mallin et al. (1999) conducted an earlier study of Hewletts Creek and Futch Creek and discovered that the phytoplankton community within these systems is very distinct.  During high tide in Futch Creek, the community was very diverse and increased phytoplankton abundance was attributed to a greater number of tiny pennate diatoms.  More flagellates characterized low tide in Futch Creek.  Hewletts Creek phytoplankton abundances were more than an order of magnitude higher at low tide than high tide and the community was dominated by flagellates and cryptomonads (Mallin et al. 1999). 

The purpose of this study was to determine how season, tide, location within the creek and watershed development impacts phytoplankton productivity in local tidal creeks.  

Methods

            Three sites were studied in both Futch Creek (FC-4, FC-6, FC-17, Fig 5.1) and Hewletts Creek (HC-2, SB-PGR, NB-GLR, Fig. 7.1).  Field sampling was conducted monthly at high ebb tide from October 2003 thru September 2004.  Beginning in March 2004 sampling was conducted monthly at both high ebb and low flood tide. 

Vertical profiles of field parameters included water temperature, pH, dissolved oxygen, turbidity, salinity, and specific conductivity.  Light attenuation was collected in situ using vertical profiles collected with a Li-Cor LI-193S spherical quantum sensor.  Total daily irradiance was logged at the UNCW Center for Marine Science, New Hanover County, NC, (Figure 2) during the week of sampling using a Li-Cor pyranometer.   

Rates of primary productivity by phytoplankton were measured using the rate of incorporation of radioactive carbon (¹⁴C).  All samples were inoculated with 0.5 ml of 4mCi NaH¹⁴CO₃.  Dark treatments were inoculated with 1.0 ml DCMU (3-(3,4-dichlorophenyl)-1,1 dimethylurea), a photosynthetic electron transfer inhibitor, to account for nonphotosynthetic uptake of ¹⁴C.  Duplicate light and single dark samples were incubated in situ in a Plexiglas bottle suspension rack for 3 to 4 hours, centered on local noon.  After incubation, all samples were filtered individually and filters were placed into separate glass vials containing 10 mL of scintillation cocktail.  Samples were radioassayed by liquid scintillation counting.  Total primary productivity was determined from the equations of Wetzel and Likens (2000). 

Phytoplankton biomass was determined via chlorophyll a pigment analysis, a fluorometric technique (Welshmeyer 1994). Phytoplankton samples were collected during spring and summer at low and high tide and field preserved with Lugol’s iodine. 

Nitrate and orthophosphate were analyzed using a Technicon AutoAnalyzer III following EPA protocols.  Ammonium was analyzed according to the methods of Parsons et al. (1984).  Dissolved inorganic carbon (DIC) was analyzed using a Shimadzu TOC-5050A total organic carbon analyzer. 

Results

            Mean annual production in Hewletts Creek was approximately 521 gC m^-3 and 257 gC m^-3 in Futch Creek.  Temporally, productivity was significantly higher during late spring and summer months than during winter months (Fig. 1).  Peak productivity corresponded with the summer chlorophyll a maximum (Fig. 2).  There was high spatial variability between sites in Hewletts Creek, however, productivity was only significantly higher in the north branch of Hewletts Creek during summer months when compared to downstream reaches.  There was no significant spatial variability between sites in Futch Creek.  The site with the highest mean daily productivity at high tide was site NB-GLR (1,348 mgC m^-3 day^-1).  The site with the lowest mean daily productivity at high tide was site HC-2 (216 mgC m^-3 day^-1).  Productivity was higher at low tide in both creek systems.  There was a decrease in high tide productivity directly following Hurricane Charley in August of 2004, which was most likely a result of decreased water column light caused by high turbidity directly following Hurricane Charley (Fig. 1).

            Phytoplankton biomass, as chlorophyll a, was significantly higher in Hewletts Creek (5.8mg l^-1) than Futch Creek (1.7mg l^-1) (Fig. 2).  The highest mean chlorophyll a concentration at high tide occurred at site NB-GLR where algal blooms (>25mg l^-1 of chl a) occurred in May and June of 2004.  Algal blooms were present at site SB-PGR at low tide during the months of April, May and June of 2004.  Phytoplankton biomass was significantly higher during summer months than during winter months.  Spatially, biomass in the upper reaches of Hewletts Creek was significantly higher than the downstream portions.  There was no significant spatial variation in chlorophyll a biomass in Futch Creek.  Mean chlorophyll a concentrations were significantly higher at low tide when compared to high tide in both creeks. 

Peak dissolved oxygen concentrations were present during the winter months and began to decline during spring.  There were no incidents of hypoxia (<2mg l^-1) in surface or bottom waters at high tide during the sampling year; however, there was one incidence of hypoxia at site SB-PGR at low tide in August 2004 (1.3 mg l^-1).  It should be noted that in addition to Hurricane Charley, a spill of over 100,000 gallons of raw sewage occurred in Hewletts Creek at site SB-PGR in the month of August 2004.

General trends indicate increased nutrient concentrations in the upstream portions of both creeks.  Nutrient concentrations were elevated in Hewletts Creek, especially at site SB-PGR, during the sewage spill in August 2004.  Mean ammonia concentrations were higher in Hewletts Creek (64.4 mg l^-1) than Futch Creek (27.3 mg l^-1).  The ammonia concentration during the sewage spill at site SB-PGR at high tide sampling was 1303 mg l^-1 and the concentration at low tide was 1,418mg l^-1.  Low tide ammonia concentrations were higher than high tide at all stations sampled.  Similar to ammonia, nitrate-nitrite concentrations were higher in Hewletts Creek (68.1 mg l^-1) than Futch Creek (49.7mg l^-1) at high tide. Elevated nitrate was present in Hewletts Creek at site SB-PGR (135.0 mg l^-1) during the sewage spill in August 2004.  Nitrate concentrations at low tide were generally higher than high tide.  Orthophosphate concentrations were also higher at high tide in Hewletts Creek (23.3 mg l^-1) than Futch Creek (16.9mg l^-1) and higher at low tide compared to high tide.  Peak orthophosphate concentrations in Hewletts Creek occurred during August 2004. 

Figure 1.  Mean monthly primary productivity versus temperature at high tide in Hewletts Creek (HC) and Futch Creek (FC), October 2003 – September 2004.  

Figure 2.  Mean monthly primary productivity versus chlorophyll a at high tide in Hewletts Creek (HC) and Futch Creek (FC), October 2003 – September 2004.  

Discussion

     Preliminary data suggest that the characteristic physical environmental forces (temperature and light) govern basic seasonal, temporal and tidal patterns in phytoplankton production.  However, greater anthropogenic nutrient loading did lead to greater phytoplankton productivity in Hewletts Creek, the more developed watershed.  Major weather events, such as Hurricane Charley can have a pronounced effect on key ecosystem processes, in this case depressing phytoplankton productivity in summer, when productivity would usually be elevated.

Table 1.  Annual primary production by phytoplankton for Hewletts Creek and Futch Creek as compared to rates in other coastal NC systems (Mallin 1994).
_____________________________________________________________________
Beaufort Estuaries                            56 gC m^-3
Neuse River Estuary                         75 gC m^-3
Futch Creek                                      91 gC m^-3
South River                                        144 gC m^-3
Pamlico River Estuary                      150 gC m^-3
Hewletts Creek                                  246 gC m^{-3
_______________________________________________________________________________}

A comparative analysis of annual primary production by phytoplankton in Futch Creek and Hewletts Creek with other neighboring North Carolina estuaries suggests that these systems could play an important role in coastal ecological processing (Table 1).  However, these creeks have not suffered from major algal bloom problems.  Phytoplankton biomass and productivity can be greatly reduced due to grazing by zooplankton, shellfish and other predators.  Models of bivalve filtration rates in shallow waters predict that sufficient numbers of bivalves can control phytoplankton biomass.  A study of bivalve populations in Hewletts Creek demonstrated 10-25% decreases in chlorophyll a concentrations as water flowed over oyster reefs, especially in summer months when phytoplankton biomass was high (Cressman et al. 2003).  While consumption rates on phytoplankton were not studied in this project it is important to note that grazing by oysters and other predators could have a significant effect on primary production by phytoplankton in local tidal creeks. 

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