Near-surface seawater from the northeastern subarctic Pacific was incubated on deck for 8 d, supplemented with (1) control, no additions (2) ϩZn (3) ϩFe (4) ϩZnϩFe. Concentrations of total Zn and Fe at time zero (t 0 ) and in the control remained at ϳ0.1-0.2 nmol L Ϫ1. In the control, chlorophyll (Ͻ0.3 mg m Ϫ3 ), 14 C uptake into POC and PIC, and inorganic nutrients all remained relatively constant. Addition of Zn slightly but significantly increased chlorophyll (p Ͻ 0.05), decreased phosphate (p Ͻ 0.01) and nitrate (p Ͻ 0.05), and in P versus E experiments, increased P m Ͼ10-fold and P 2-3-fold. The abundance of small diatoms and coccolithophores was higher in the chl m ϩZn treatment compared to the control. The ϩFe and ϩZnϩFe treatments, compared to the control, both showed Ͼ10-fold increases in chlorophyll and 14 C uptake into POC and PIC and complete removal of nitrate (Յ0.2 mmol m Ϫ3 ). However, differences were observed in size-fractionated data; the ϩZnϩFe treatment had significantly lower percent chlorophyll in the Ͼ20-m fraction (p Ͻ 0.01) and a higher percentage in the 0.2-5-m fraction (p Ͻ 0.01) than the ϩFe treatment. In P versus E experiments, both ϩFe treatments increased P m and ␣ around 100-fold and P and ␣ chl by 5-10-fold compared to the control. The ϩFe treatment showed a slightly higher ␣ chl and slightly chl m lower P than the ϩZnϩFe treatment. Abundance of large diatoms, small diatoms, small flagellates, and coccolchl m ithophores all increased substantially (ϳ7-1,000-fold) in response to Fe addition, whereas dinoflagellate abundance only doubled. The ϩZnϩFe treatment had higher abundances of small diatoms and small flagellates than the ϩFe treatment. We conclude that Zn additions had limited influence on conventional indices of phytoplankton growth compared to Fe, but that there might be subtle influences of Zn that require further attention.

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Microzooplankton, on average, ingested 10 times more phytoplankton productivity than Acartia tonsa, which dominates the mesozooplankton community in Apalachicola Bay, Florida, USA. Microzooplankton ingested < 75 and > 75% of phytoplankton productivity during winter and summer, respectively. Ciliates, particularly ciliates < 20 μm in size, were abundant in all samples. Phytoplankton was the main (> 50%) component of the microzooplankton diet during the summer high productivity period. Because microzooplankton are prey for organisms occupying higher trophic levels, such as copepods, fish larvae, and oysters, we propose that energy and mass flows mainly from phytoplankton through microzooplankton to higher trophic levels in Apalachicola Bay. A reduction in discharge from the Apalachicola River during our study period led to reductions in rates of grazing, ingestion, and productivity of microzooplankton at a particular salinity. Reduced river discharge also increased the areal extent of higher salinity water where ingestion and productivity of microzooplankton were relatively low. Because microzooplankton are key constituents of the estuarine food web in Apalachicola Bay, upstream water diversion that increases the areal extent of higher salinity water in the bay, can be expected to lower microzooplankton productivity and therefore reduce higher trophic level productivity in the bay.KEY WORDS: River flow · Management · Estuarine · Food web structure · Microzooplankton Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 345: [63][64][65][66][67][68][69][70][71][72][73] 2007 ture in Apalachicola Bay. However, microzooplankton are abundant in estuarine waters (Gifford & Caron 2000). Furthermore, recent studies indicate that microzooplankton are generally the primary herbivores in oceanic, coastal, and estuarine waters . Microzooplankton are also prey to higher trophic levels such as copepods (Rollwagen Bollens & Penry 2003), oysters (Dupuy et al. 1999), larval shellfish (Nagano & Decamp 2004) and larval fishes (Fukami et al. 1999) that inhabit estuaries. Therefore, the classical diatom-copepodfish food chain, where copepods are the main secondary producers, may not be an accurate representation of mass and energy flow through the planktonic food web in Apalachicola Bay or other estuaries.The first objective of the present study was to test the hypothesis that microzooplankton are the primary herbivores in Apalachicola Bay. If microzooplankton are the main herbivores, then the percent of phytoplankton productivity ingested by microzooplankton will be greater than that ingested by mesozooplankton. Data obtained from a parallel study of mesozooplankton grazing were used to test this hypothesis (Putland & Iverson 2007). The second objective of this study was to examine relationships between microzooplankton parameters and salinity in the bay, specifically during summer when river diversion has been proposed. We chose to examine parameters with respect to salinity as ...