Latest
Monitoring Results:
Selected
results from the monthly monitoring program. The color
scheme adopted for all figures is as follows: green
= spring, red = summer, orange = fall, blue = winter.
Filled symbols are samples collected during flood
tide, open or stippled symbols are ebb-tide samples.
During two months (Sept 02 & Mar 03) samples were
collected during storm conditions (tropical storm
and Nor'easter respectively) as indicated by symbols
with heavy black outline - these are typically the
highest values seen, indicating the importance of
natural events in driving extreme values
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Fig
1: Animation of extent of tidal influence into North
River, from the nearby Beaufort Inlet, courtesy of
Dr. Leuttich (http://www.marine.unc.edu/C_CATS/sabre/sabre.htm).
Tidal currents are directed along deep channels between
Middle Marsh. The results in distinct water masses
that mix with tidal exchange, such that there are
three broad zones of water quality in North River,
as indicated on the map on the previous
page.
Fig 2: Nitrogen and Phosphorus concentrations (ug/L) measured in North River during monthly cruises. N typically increases from low concentrations near Beaufort Inlet (MM1) upstream to the Highway 70 Bridge (MM9). Higher concentrations of N (primarily ammonia) were recorded in the late summer and early fall of 2002, after a period of drought ended with heavy rainfall, resulting in runoff from nutrient rich sources nearby. N-concentrations during the much rainier period of spring and summer 2003 were correspondingly lower due to greater dilution of nutrient sources.
Fig 3: Chlorophyll concentrations (ug/L) were lowest during the cold winter months and increased during the spring, summer, and fall when phytoplankton are more abundant. Highest values were seen at the farthest upstream station (MM9) corresponding to increased N-concentrations. High chlorophyll concentrations were also recorded during storm events, likely due to resuspension of benthic microalgae. Even so the measured concentrations were well below the NC-State acceptable criterion of 40ug/L chl a for estuarine waters.
Fig 4: Turbidity was measured as total suspended solids (TSS) expressed in mg/L. Turbidity was similar at all stations, and only slightly lower in winter than other seasons. The highest turbidity values were recorded at shallow sites during storms (e.g. MM9, Sept 02 & Mar 03). Except during these conditions, turbidity did not exceed the NC-State acceptable criterion of 25NTU (= 68mg/L TSS) for estuarine waters. The relationship of TSS to NTU was derived by linear regression and is approximately NTU = 0.368[TSS], r 2 =0.986. For reference, the federal drinking water standard is 0.5NTU and water is visibly cloudy at 5NTU.
Fig 5: The effect of chlorophyll and turbidity on measured light penetration to the bottom at each of the nine stations. Light attenuation is expressed as an attenuation-coefficient (Kd) derived from the negative exponential Beer-Lambert function. Higher values of Kd indicated more light attenuation (i.e., more turbid-looking water). Kd values increased with distance from the Beaufort Inlet station (MM1), with highest values found at MM9, where chlorophyll and turbidity were typically highest. For seagrass survival Kd should not exceed 1.35, i.e. 25% of surface light reaches the bottom (black bar). This condition is typically met at the stations where seagrass are found in North River (MM2-MM6, green arrow indicates relative abundance of healthy seagrass beds). Kd values sufficient for seagrass survival are rarely seen in the upstream stations.
