Sirkulasi Laut dan Implikasinya Terhadap Persebaran Larva Ikan di Teluk Cendrawasih - Papua

Abstract

Teluk Cendrawasih (TC) merupakan salah satu teluk terbesar di Indonesia dan merupakan teluk semi tertutup yang memiliki biodiversitas hayati laut tinggi. Lokasi TC yang termasuk di dalamnya teluk dalam, Selat Biak dan teluk luar (Samudera Pasifik), berada dekat ekuator dan berhadapan dengan Samudera Pasifik ekuator barat diduga memiliki dinamika dan variabilitas laut yang unik. Tujuan dari penelitian ini adalah menganalisis dinamika arus laut, biogeokimia dan variabilitas volume transpor, serta analisis persebaran trajektori transpor larva ikan di kawasan TC. Dataset deret-waktu yang digunakan keluaran model sirkulasi laut dari Infrastructure Development of Space Oceanography (INDESO) dari periode 2008-2015 dan simulasi model Coastal and Regional Ocean Community Model (CROCO) dari periode 2013-2014. Pola sirkulasi permukaan di luar teluk dicirikan dengan pembalikan arah arus dua kali dalam setahun yang sejalan dengan pembalikan angin muson, sedangkan di dalam TC arus permukaan mengalir keluar dari teluk saat musim Timur dan membentuk pusaran arus (eddy) saat musim Barat. Sirkulasi di kedalaman 110 m bersifat persisten dan permanen Arus Pantai Papua (APP) yang mengalir kearah barat laut, serta terbentuk pusaran arus pada musim Timur. Arus monsun terlihat sampai kedalaman 50 m, sedangkan APP berada pada kolom dari 50 m sampai 200 m (di Selat Biak) dan sampai 1000 m (di sisi Samudera Pasifik) dengan kecepatan maksimum 0.75 m s-1. Rerata dan simpangan baku dari volume transpor APP adalah -25.25 (±11.10) Sv (1 Sv = 106 m3 s-1), sedangkan di Selat Biak -0.87 (±0.50) Sv. Variabilitas dari volume transpor APP didominasi periodisitas tahunan dan terlihat adanya skala-waktu antar-tahunan (2.8 tahun) dan intra-musiman (64 hari). Rerata volume transpor di pintu masuk barat TC adalah 0.03 (±0.11) Sv dimana variabilitasnya domianan pada periode intra-musiman (9 dan 14 hari). Koherensi antara suhu air laut kedalaman 150 m mencapai 0.9 di kawasan ekuator Samudera Pasifik, dengan fluktuasi suhu pada variabilitas skala-waktu inter-tahunan. Selain itu, koherensi antara arus zonal dengan angin zonal Indo-Pasifik juga menunjukan koherensi mencapai 0.76 di wilayah selatan ekuator Samudera Pasifik pada skala inter-tahunan. Variabilitas arus zonal dijelaskan dari empat metode utama dengan jumlah explained variance sebesar 84.88%. Pola spasial dalam mode pertama dengan varians sebesar 50.22% menunjukan nilai negatif di wilayah Samudera Pasifik dan Selat Biak, diduga merupakan arus APP yang bergerak ke arah barat. Pola spasial mode 2 dan 3 diduga menunjukan arus APP permukaan yang bergerak kearah timur dengan dengan memperlihatkan nilai negatif di dekat permukaan Samudera Pasifik dan Selat Biak. Sedangkan untuk pola spasial mode 4, diduga menunjukan arus APP yang mencapai kedalaman 300 m dan arus dasar yang berlawanan arah di wilayah Samudera Pasifik. Variabilitas temporal dari arus zonal hasil analisis energi densitas EOF mode 1 sampai mode 4, menunjukan dominan siklus tahunan yaitu 341 hari. Konsentrasi nitrat (NO3) di TC dan Samudera Pasifik memiliki nilai mencapai 0.00103 mg l-1 dan 0.05930 mg l-1; konsentrasi fosfat (PO4) di TC dan Samudera Pasifik masing-masing memiliki nilai 0.00840-0.02662 mg l-1 dan 0.00832-0.02965 mg l-1; Konsentrasi silikat di TC dan Samudera Pasifik masing-masing memiliki nilai 0.11320-0.22550 mg l-1 dan 0.02309-0.10050 mg l-1. Rata-rata produktivitas primer bersih (NPP) di TC adalah 599.6 (±55.76) mgC m-2 day-1, sedangkan di Samudera Pasifik sebesar 499.7 (±139.5) mgC m-2 day-1. Variabilitas dari parameter biogeokimia di TC dan Samudera Pasifik didominasi periodisitas antar-tahunan, tahunan dan intra-musiman. Model transpor larva ikan dilepas pada dua bulan yang berbeda (April & Oktober) dan di 6 zona yang berbeda di kawasan TC. Simulasi bulan April pada zona 1-6 menunjukkan partikel larva ikan banyak ditemukan di wilayah selatan TC, sisi Timur TC, dekat pulau Yapen dan Pulau Biak. Simulasi bulan Oktober pada zona 1-6 menunjukkan partikel juga banyak ditemukan di sisi barat teluk, timur teluk, Pulau Yapen dan Pulau Biak. Simulasi transpor larva pada bulan April dan Oktober di dalam teluk banyak dipengaruhi oleh adanya pusaran arus searah maupun berlawanan arah jarum jam dan juga adanya pengaruh arus NGCC yang membawa masuk partikel kadalam teluk. Hasil simulasi menunjukkan pada bulan Oktober partikel lebih banyak yang death dibandingkan bulan April. Hasil model trajektori larva tersebut dapat diprediksi wilayah pemijahan dan ikan dewasa dari ikan pelagis kecil

Cendrawasih Bay (TC) is one of the largest bays in Indonesia and is a semi-enclosed bay that has high marine biodiversity. The location of the TC which includes the inner bay, Biak Strait and the outer bay (Pacific Ocean), which is near the equator and facing the western equatorial Pacific Ocean is thought to have unique ocean dynamics and variability. The purpose of this study was to analyze the dynamics of ocean currents, biogeochemistry and transport volume variability, as well as to analyze the distribution of fish larvae transport trajectories in the TC area. The time-series dataset used is the output of the ocean circulation model from the Infrastructure Development of Space Oceanography (INDESO) from the 2008-2015 period and the Coastal and Regional Ocean Community Model (CROCO) simulation from the 2013-2014 period. The time-series dataset used is the model output. ocean circulation from Infrastructure Development of Space Oceanography (INDESO) from 2008-2015 and Coastal and Regional Ocean Community Model (CROCO) simulations from 2013-2014. The results of the study revealed that the surface circulation pattern outside the bay is characterized by reversal of the current direction twice a year which is in line with the reversal of the monsoon winds, while inside the TC surface currents tend to always flow out of the bay and form eddies in the West monsoon. Circulation at a depth of 110 m is persistent and permanent, the Papua Coastal Current (APP) flows to the northwest, and forms eddy currents in the east monsoon. Monsoon currents are seen to a depth of 50 m, while APP is present in columns from 50 m to 200 m (in the Biak Strait) and up to 1000 m (on the Pacific Ocean side) with a maximum speed of 0.75 m s-1. The mean and standard deviation of APP's transport volume is -25.25 (±11.10) Sv (1 Sv = 106 m3 s-1), while in the Biak Strait it is -0.87 (±0.50) Sv. The variability of APP transport is dominated by annual periodicity and inter-annual (2.8 years) and intra-seasonal (64 days) timescales are seen. The mean transport volume at the west entrance of the TC was 0.03 (±0.11) Sv where the variability was dominant in the intra-seasonal period (9 and 14 days). The coherence between seawater temperatures at a depth of 150 m up to 0.9 in the equatorial Pacific region, with temperature fluctuations in the inter-annual time-scale variability. In addition, the coherence between zonal currents and Indo-Pacific zonal winds also shows a coherence of 0.76 in the south equatorial Pacific Ocean region on an inter-annual scale. The variability of the zonal current is explained from the four main methods with the total explained variance of 84.88%. The spatial pattern in the first mode with a variance of 50.22% shows a negative value in the Pacific Ocean and Biak Straits, thought to be the APP current moving westward. The spatial pattern of modes 2 and 3 is thought to indicate the APP current moving eastward by showing negative values near the surface of the Pacific Ocean and the Biak Strait. As for the spatial pattern of mode 4, it is suspected that it shows the APP current reaching a depth of 300 m and the bottom current in the opposite direction in the Pacific Ocean region. The temporal variation of the zonal current from the energy density analysis of EOF mode 1 to mode 4, shows the dominant annual cycle of 341 days. The concentration of nitrate (NO3) in the TC and the Pacific Ocean has a value of 0.00103 mg l-1 and 0.05930 mg l-1; concentrations of phosphate (PO4) in the TC and Pacific have values of 0.00840-0.02662 mg l-1 and 0.00832-0.02965 mg l-1, respectively; The silicate concentrations in the TC and Pacific Ocean have values of 0.11320-0.22550 mg l-1 and 0.02309-0.10050 mg l-1, respectively. The average net primary productivity (NPP) in TC was 599.6 (±55.76) mgC m-2 day-1, while in the Pacific it was 499.7 (±139.5) mgC m-2 day-1. The variability of biogeochemical parameters in the TC and the Pacific is dominated by inter-annual, annual and intra-seasonal periodicity. The fish larvae transport model was released at two different months (April & October) and in 6 different zones in the TC area. Simulations in April in zones 1-6 showed that fish larvae were mostly found in the southern region of TC, east side of TC, near Yapen and Biak Islands. Simulations for October in zones 1-6 show that particles are also found on the west side of the bay, east of the bay, Yaprn Island and Biak Island. The simulation of larval transport in April and October in the bay was heavily influenced by the presence of clockwise and counterclockwise currents and also the influence of NGCC currents that brought particles into the bay. The simulation results show that in October, more particles died than in April. The results of the larval trajectory model can predict the spawning area and adult fish of small pelagic fish.